Hi and thank you for sharing your opinion!
Nothing can get better if nobody is aware of the problem.
However; i don’t fully agree with your post in its entirety.
The silencer and barrel extension are both “operational”, the silencer does make each bullet quiet, making it possible to take out individual enemies without alerting the group.
This seems to be broken sometimes since the enemies do react to (some) incoming fire and can actually gauge the direction of said fire, but i do think that this is the proper behaviour, a few gyro´s and accelerometers could in theory detect the force and direction of the impact and could therefore in theory be used to calculate the origin of the incoming rounds.
The barrel extensions do what they’re supposed to, increasing bullet velocity with reduced bullet drop and increased accuracy as a result.
The compensator reduces recoil and its my preferred muzzle attachment. 
And the handgun compensator does indeed fit a handgun, the .44 accepts it.
I do agree with you, if you can fit ANY muzzle attachment to the gun, you should be able to fit them all.
In other words; the glauke can accept silencer, but it doesnt take a compensator, witch doesnt make even the slightest ammount of sense.
A (screw-) thread is a thread and it will accept any properly sized threaded item regardless if its a silencer or a screw (if its internal thread, else it would accept a bolt).
I don’t particularly like the empty attachment slots on the weapons that cant accept that type of attachment, but i can imagine that this is a design-choice, having all the weapons look and feel identical in the inventory menu.
Instead of removing the empty slots, one could color them red to highlight that this particular weapon does not accept that particular attachment type.
And i can relate to the desire to have drum mags on my glock, a handle in front of the magazine on assault rifles and so on, but we must remember the core idea in this game.
We are using standardized military weapons from the late 80´s, none of witch had drum magazines and only a few variants of the stock of the assault rifles (shorter version + collapsable version and the standard length stock).
Ive never seen a front grip that came from the swedish military, for each and every time ive seen it, its the soldier himself who ordered it off ebay or similar.
This might have changed now, due to the increase in overseas missions (like in the middle east and former jugoslavia), but i doubt you´dd find anything non-standard (except possibly stickers) on any goverment issued weapons from that late 80´s era.
The red dot sight is my main agonizer; the red dot itself is a bit to big, just as you described, and secondly (but more importantly!) its also a magnification of 2x!
No red-dot sight ive ever tried has any magnification whatsoever, the sight is designed to be able to be used with two (2!) open eyes, one sighting through the red dot, the other sighting along the weapon side, to further increase situational awareness and combat effectiveness when aiming.
And finally, i want to contribute and not merely argue
So i think that we should be able to fabricate (or modify) attachments and weapon mods, building a front grip out of a broom, a silencer from a car muffler and so on.
My reasoning:
IF my government falls and im left to fend for myself and there´s military weapons all over the place, i would definitely go to town on those weapons with duct tape and nails, to build myself DIY-silencers, bipods, front grips and what else i could find useful for my combat effectiveness.
I would not hesitate to bring out a axe or saw to customize the stock of the rifle, if i deemed it favourable for my chances of survival.
Taping together 2 magazines into a “JägarMag” (“hunters”/rangers magazine) with a spent ammunition casing in between them (to simplify and speed up changing of mags) is a very good example of this human behaviour; if your equipment isn’t good enough, you´ll find ways to improve it.