Does this spell get impacted by draconic bloodline sorcerers at level 6 if there affinity is fire?

I would strongly doubt it as the spell does not directly cause fire damage - any fire damage from burning webs is incidental.  Put another way, the fire damage from burning webs is not magical fire damage, and thus would not be affected by the Sorcerer's Elemental Affinity feature any more than if the forced movement of a Gust of Wind spell pushed something into contact with a source of fire and caused it to take fire damage.  

If a creature in the middle of the 20' cube breaks free of the web and in the same turn moves into another 5 feet of web, does this trigger another Dex save?   In other words, is navigating the web considered "entering the webs" for every 5 feet covered by it?

I was just considering this as my wizard just picked up the spell. Here's my interpretation:

When a creature enters the space or starts in it they make a Dex save, failure means they're restrained. I think if they succeed they should be able to move through the webbing, still considering it is difficult terrain (double movement cost). If they don't make it out they would have to do a Dex save at the start of their next turn per the spell description. Rinse, repeat. Additionally I would think succeeding the Str check after being restrained would work the same, difficult terrain movement through any further webbing the rest of the turn.

Quote from Ningauble7Eyes >>

If a creature in the middle of the 20' cube breaks free of the web and in the same turn moves into another 5 feet of web, does this trigger another Dex save?   In other words, is navigating the web considered "entering the webs" for every 5 feet covered by it?

 No, because the target already is in the web - it doesn't enter it anew.

How much would this spell weigh? im assuming that the strands are much much thicker than a normal spiderweb if it can restrain quite large creatures

It can restrain any creature that fails the inital save, and then fails the 'strength' check to break free, regardless of size of the creature.

Casting Word is:

Interlacium Fixus

I know the reasons for them not are basically historic (how it was handled in previous editions) but all druids should really get this.

Fighter 2 / Wizard or Sorcerer 5+:
Web + action surge Fireball on a Huge creature = 
For a huge creature (20x20x20) taking up the full volume of web, that yields 128d4 damage on top of whatever damage fireball itself does. :D

If you have a spider familiar, you should have this spell.  You can make a bridge for your spider to cross chasms!  Or, I guess, just throw the spider.

It says starts or enters

Yes, but you don't enter the web anew if you just move around inside it. "enter" refers to going from outside the web to inside the web

Quote from DnDDetective >>

I know the reasons for them not are basically historic (how it was handled in previous editions) but all druids should really get this.

 Correct.  Obviously both Druids and Rangers should have access to this.

You can't cast two spells in one round.

You can, as long as neither is a bonus action spell. :)

PHB, p. 202. 

'You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.'

 Question: Can I misty step (bonus), fire bolt (action) and fireball (surge) on same turn? Or need action surge also be a cantrip?

Crawford: In that situation, the Action Surge spell would also need to be a cantrip

Crawford: The rule on bonus action spells pertains to your whole turn, not to a particular action (PH, 202).

Quote from JIbdux >>

PHB, p. 202. 

'You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.'

 Question: Can I misty step (bonus), fire bolt (action) and fireball (surge) on same turn? Or need action surge also be a cantrip?

Crawford: In that situation, the Action Surge spell would also need to be a cantrip

Crawford: The rule on bonus action spells pertains to your whole turn, not to a particular action (PH, 202).

That's an incomplete quote. The full passage reads:

"Bonus Action

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action."

So this limitation ONLY applies if one of the spells you cast is a bonus action spell.

Web + Fireball via Action surge = OK
Web + Magic Stone = X
Web + Misty Step = X

In the Crawford quote, the problem is that he cast Misty Step. That's why he can't then go on to do Fireball.  

Quote from Akamoria >>

Quote from JIbdux >>

PHB, p. 202. 

'You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.'

 Question: Can I misty step (bonus), fire bolt (action) and fireball (surge) on same turn? Or need action surge also be a cantrip?

Crawford: In that situation, the Action Surge spell would also need to be a cantrip

Crawford: The rule on bonus action spells pertains to your whole turn, not to a particular action (PH, 202).

That's an incomplete quote. The full passage reads:

"Bonus Action

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action."

So this limitation ONLY applies if one of the spells you cast is a bonus action spell.

Web + Fireball via Action surge = OK
Web + Magic Stone = X
Web + Misty Step = X

In the Crawford quote, the problem is that he cast Misty Step. That's why he can't then go on to do Fireball.  

I actually never knew that the limit was applied only to bonus actions but it does make sense. Otherwise you could never counterspell a counterspell unless your original spell was a cantrip. To add another source to this I found a passage in the SA Compendium from a post on Stack Exchange.

Is there a limit on the number of spells you can cast on your turn?

There’s no rule that says you can cast only X number of spells on your turn, but there are some practical limits. The main limiting factor is your action. Most spells require an action to cast, and unless you use a feature like the fighter’s Action Surge, you have only one action on your turn. If you cast a spell, such as healing word, with a bonus action, you can cast another spell with your action, but that other spell must be a cantrip. Keep in mind that this partic-ular limit is specific to spells that use a bonus action. For instance, if you cast a second spell using Action Surge, you aren’t limited to casting a cantrip with it.

Quote from Owin >>

Does this spell get impacted by draconic bloodline sorcerers at level 6 if there affinity is fire?

Quote from dakingsman >>

I would strongly doubt it as the spell does not directly cause fire damage - any fire damage from burning webs is incidental.  Put another way, the fire damage from burning webs is not magical fire damage, and thus would not be affected by the Sorcerer's Elemental Affinity feature any more than if the forced movement of a Gust of Wind spell pushed something into contact with a source of fire and caused it to take fire damage.  

I tend to disagree here. I was looking into it for clarification about Arcane Firearm and Web. The feature specifies "one of the spell’s damage rolls" and under Combat -> Damage and Healing in the PHB it says:

Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target. Magic weapons, special abilities, and other factors can grant a bonus to damage.

 Since Web specifies the damage and die I assume that makes it a spell damage roll, and therefor spell damage. 

I can only think of 2 things that specify damage when ignited, Web and a flask of oil. They could have easily omitted the flammable part on Web so its inclusion leads me to believe that it's intentional spell damage.

In the end though talk to your DM about it.