yeah my spell called Decoy party requires an investigation roll before they attack and if they get below 15 they miss regardless

Definitely not

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

 Not sure if I would agree with that.  When fighting, you hit what you can get. It could be that the opponent exposes and arm or a leg and that is where you hit.

Yes but that's not how 5e works.

You occupy a 5ft square no matter what. If someone swings to hit you, you're size real or fake doesn't matter.

So many people don't read this spell and make some seriously out there reaches.

 So, if all anyone ever targets is a 5 foot square, why does being prone grant disadvantage to ranged attacks? And why do people adjacent to you have advantage on the attack if you're prone? They're just hitting the 5 foot square. Why does being invisible impose disadvantage for someone to hit you? You're just a 5 ft square. That's all they need to hit you, right?

So, yeah. In my games, if some lean adventurer was like "I desguise my self as morbidly obese" and fought someone, there's a full stomach there that just doesn't exist. If the oponent doesn't realize it's an illusion, they would think it's a valid target. In this particular instance, the person with the illusion wouldn't be moving the way someone of that actual size would, and I'd grant advantage on the check to realize it's an illusion, and probably come up for some way for the opponent to automatically figure it out from a failed strike. If I cut through someone's stomach thinking I just eviscerated them but felt no resistance, saw no blood, and there was no injury, I'd find out right away.

Disadvantage isn't going to break the game. It's still possible to be hit through it. And if a player "abuses it" just remember that AoE exists.

If you Disguise yourself as wearing silk pyjamas and slippers would someone have to succeed on an investigation check in order to hear you? 

since this doesn't give a specific stat bonus, its they could probably work together.

Disguise self: "To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC."

Actor feat says: "You have advantage on Charisma (Deception) and Charisma (Performance) checks when trying to pass yourself off as a different person."

Silver Tongue says "you are a master at saying the right thing at the right time. When you make a Charisma (Persuasion) or Charisma (Deception) check, you can treat a d20 roll of 9 or lower as a 10."

Disguise kit "lets you add your proficiency bonus to any ability checks you make to create a visual disguise."

It seems insane that none of these other three abilities are actually relevant when using disguise self??  Am I missing something?

Would it be worth being a plasmoid and disguising myself as a mimic?

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Disguise self: "To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC."

 

Actor feat says: "You have advantage on Charisma (Deception) and Charisma (Performance) checks when trying to pass yourself off as a different person."

Silver Tongue says "you are a master at saying the right thing at the right time. When you make a Charisma (Persuasion) or Charisma (Deception) check, you can treat a d20 roll of 9 or lower as a 10."

Disguise kit "lets you add your proficiency bonus to any ability checks you make to create a visual disguise."

 

It seems insane that none of these other three abilities are actually relevant when using disguise self??  Am I missing something? 

Well, the Disguise Kit is for creating mundane non-magical disguises, so it wouldn't really make sense for it to apply to a magical illusion.

As for the others, I think the important thing here is that there's a difference between actively trying to convince someone that you're someone you're not, and someone else actively trying to figure out whether you are who you say you are. 

Here's an example. Say you've used this spell to magically disguise yourself as the Mayor of whatever town you're in. If you go into the Mayor's office looking like the Mayor and tell her assistant that you need 1000 gold out of the city's treasury right away, the DM will probably have you make a Persuasion check. That check could reasonably benefit from both Actor and Silver Tongue. 

Let's say you get your gold and are making your escape but word gets out that there's a fake Mayor running around. A guard stops you and tries to judge whether you're the real Mayor or the fake one. In this situation, the guard will make an Investigation check against your spell save DC as noted in the spell description.

Don't take this as a hard and fast rule, it's just how I would interpret this stuff as a DM.

this spell always felt useless to me. The effect is good but the fact it only lasts an hour makes it seem like it's useless for long-term applications or infiltration. Invisibility is a second level spell and lasts just as long, and makes it easier to infiltrate a place since you don't also need to talk your way in. In most cases it seems like a disguise kit would be more practical. The only time I'd use this is to divert the attention of someone for a short time. If it lasted 8 hours then I could at least use it to have a long-term disguise and actually play the long game, it just seems outclasses by every other option out there.