For reference I am a old school 2nd ED AD&D TSR guy I am 43. So I been around this since I was 12.

So why did the scimatar go from D8 in 2nd Ed to D6 in 3rd/Pathfinder? Oh and why isn't a scimatart a finesse available weapon?

Why Are falchions two handed? The most most famous mueseum falchion is a single handed blade IE the Thorp Falchion.

Also traditionally it was a side arm weapon for english achers who used it like a machete/clever to chop up french knights who tried to charge the english long bow.

As for the dualing sword which IMO is modeled after the messer and or Lange messer. Is there a companion blade that goes with it? IE Long Sword & Shor swordt, Estoc/Rapier & Main Gache

How come there isn't any coat of plates and or Brigdine armor in the heavy armor category?
How come 1/2 plate has no dex modifier yet full plate does?

high crit range. It can be finessed with feats. why arming swords are called longswords? because DnD is stupid when it comes to gear and always was. It also suggests leather armor is light and silent and is more protective than padded, LoL. Again, doesn't matter. historical accuracy(or common sense) and DnD are as disconnected as humanly possible. Any light blade will do if we are talking mechanics. Or you would need fighters effortless dual-wielding(if you have varn dlc, I think) As to armor variations, coat of plates and brigandine are not in core rules iirc and wouldn't mechanically much differ from scale-mail and half-plate(or full-plate if you are generous). 1/2 plate just serves to be a bad(but cheap) full plate in 3e+.

If I recall correctly there were a lot of mechanically equal weapons in 2e, for instance with the longsword and the scimitar both being 1D8, both with a speed factor of 5.

In later editions they wanted to give different weapons more of an own identity (and speed factors went away). So, amongst others, they give weapons different critical hit ranges and damage multiplier's. A longsword scores a critical hit not only on a 20 but also on a 19.
The scimitar (and it's piercing counterpart the rapier) was thought of as getting more critical hits (18, 19 and 20), but dealing slightly less base damage.
You see the same thing with twohanded weapons. Greataxes (20/x3), Greatswords (19-20/x2) and Falchions (18-20/x2 but less base damage). I guess they thought this was the best way to make the falchion stand out.

what's not in the game is not in the game. In case of brigandines and half-plate you can write a letter to paizo and argue with them all you want.
I, myself, rename/reskin everything when I DM, because it's stupid, but PK uses mostly errata rules(and even those are not implemented fully and not always correctly), so take it or leave it, I guess.
Also, full brigandine required about as much labor to produce as a full plate. It's just a later period armor.