Can one make plate armor with this spell?

if enough material is there to make the full set and the caster is proficient in blacksmith tools then yes. it would still take a good bit of coin to purchase the material required to craft it. The metal being most of the cost. 

If you had proficiency with smiths tools and/or have the guild artisan background. 

If I'm proficient with alchemist tools and have lit a diamond on fire before and performed other experiments on it, could I make diamonds out or trees of coal with this?

Perfect for an artificer.

If you had an animal such as a pig, could you fabricate it into a banquent?

Quote from PearlsBefore4Swine >>

If you had an animal such as a pig, could you fabricate it into a banquent?

 
For a 4th level slot I'd allow it, as long as it was already dead and you were proficient with Cook's Utensils. Just remember a lot of plant matter isn't dead when you start to cook with it, so adjust the recipe as needed.

Can we use multiple materials in the same Fabricate spell? Or would the spell caster need to cast it twice, once for the handle, once for the head?
For example: wood + metal to make a simple Axe (given the smith's tools proficiency)

As a DM I would either a) require two roll for each part (axe head and the the handle) or b) if they have the skill required make a roll against that also.

 I would like to have seen this as a weaker first level spell, that scales with higher level slots.

Quote from Farva9196 >>

if enough material is there to make the full set and the caster is proficient in blacksmith tools then yes. it would still take a good bit of coin to purchase the material required to craft it. The metal being most of the cost. 

 Im pretty sure iron is dirt cheap

Quote from brobjorn >>

Can one make plate armor with this spell?

 If you have the materials and are proficient with smiths tools

Quote from Melsin >>

If I'm proficient with alchemist tools and have lit a diamond on fire before and performed other experiments on it, could I make diamonds out or trees of coal with this?

 Id allow it if youre proficient with jewelers tools.

But youd need coal, probably.

Which you could make from trees. :-p

'

The usage of this spell is really only limited by your creativity. Last week I used it to burn a legendary resistance off a flying dragon, by fabricating a stone cube above it to drop on top of it.

How did you do that with a casting time of 10 minutes? You weren't in combat yet? 

Was allowed at DM's discretion. My character used magic to solve problems in all sorts of unconventional ways, so fabricate was allowed as an action. My character hardly used any damaging spells, so certain things were allowed.

can you use it on inanimate objects attached to a creature. like making a wool sweeter well it is still attached to the sheep? or transforming someones armor well there wearing it?

Depends on how romanticised your definition of a "creature" is. The spell reads that "creatures can't be transmuted by this spell," so, considering the wording on other spells, ask yourself if teleporting eight creatures carries their hair & fingernails with them.

If a mundane steel door is encountered, could you fabricate it into 1x1 ft metal chunks that could be easily carried away? Would this work with higher level materials?