Would the Warlock's ability to cast this at will synergize with the Actor feat, or would they be mostly redundant?  

If a thin character like an elf would use this spell to take the appearance of a really fat person. Would his attackers have disadvantage hitting him untill they succeed on the Intelligence check?

It actually would be a great combo (one I'm considering myself)

The spell allows you the look like someone, but you still sound and act like yourself

The feat allows you to mimic that person's voice and have a easier time visually acting like them

Together you would have an easy time passing yourself off as practically anyone as long as no one touches you (would need a higher level spell, like polymorph, for that)

@ottaviani

It would be up to the DM, but I would say that most DMs would agree with that, but I would rule that if they missed only by a little bit, they would get advantage on realizing it's an illusion (or out-right dis-spell the illusion, similar to attacking a mirror image), since they would probably see their weapon go throw you with no seeming effect or resistance

Alter self would solve the touching problem and is still only 2nd level

When playing as a warlock and take the mask of many faces do I have to get the spell myself of does it come with it?

It adds it to your spells known. If you use the DnDbeyond builder, it adds it to your spell list automatically as an 'at-will' spell

Casting Spellword is:

Shiftus Palasium 

But Alter Self is a concentration spell, which can be inconvenient.

I mean, you could absolutely make that argument, but man that's reeeeeally against the spirit of the rules and the intent of the spell.

No. Attacks are aimed at the core of a target, not the periphery.

If my character was wearing an old rusted breastplate armor, and used the spell to make it look shiny and new with gold patterns and jewels, then gave the armor to an NPC would it immediately change to the bad rusted armor as soon as it leaves my hand since it is not on  MY person, or would it change after concentration breaks, an hour has passed, or the NPC succeeds in investigation?

Not sure what I'm missing here but the new Ranger Class archtype, Gloom Stalker, gives the Disguise Self spell. However, on DnDbeyond, the do not allow this spell in the selections.

What am I missing?  How do add that spell to my Ranger?

It will be automatically added. The additional spells in the Ranger subclasses are known as soon as the ranger reaches that level. Dnd beyond builder will add them automatically!

If it is cast at a higher level (2nd+), does it have any additional benefits like a longer duration, more solid/realistic, or anything?

Pachilles: No, unless a spell states an upcast benefit, there is none. 

Seems interesting that this doesn't require concentration, especially for the Warlocks who can use it at-will. Lets you combine it with charm spells for huge effect that would be blocked otherwise. 

Thanks!

Makes sense.  I agree.

dose anyone's DM make them roll a DC to see how well the disguise self spell was performed?

Could a Trickster cleric use this in tandem with their Mirror Image spell at lvl 2 since this is not a concentration spell?

Essentially, cast this to disguise yourself as a different character, then cast friends on a creature to get information, afterwards, run away as fast as you can and transform back into yourself once you are out of sight.