1) The A4 has been legal for service rifle competition since 2004. Just make sure you use a carry handle for rear sight. No BUIS, no optics. A3 is a fully automatic A2, not a flat top, regardless of what Bushmaster says.
2) If you are not going to float the barrel, go HBAR. an HBAR will be affected by sling tension, but a GI profile will be affected significantly more. Thicker is stiffer.
3) There are plenty of match grade barrels made of 4140 Chrome-Moly steel. The steel type is not what determines if a barrel is match grade or not. The stress relieving of the blank, the dimensional consistency of the rifling, the lack of induced stresses while rifling it, and the quality of the chambering are what makes a barrel "match grade".
4) A 20" 1/9 will stabilize 75 grain bullets, BTDT. However, if you are going to buy a barrel, why limit yourself? A 1/8 or 1/7 will stabilize anything a 1/9 will, while also allowing you to use 80 grain bullets (which a 1/9 will NOT stabilize reliably).
Now that I answered your questions, answer me one.
Why would you buy/build a compromise upper (what you just described) when for the same money you can buy a Rock River NM A4 upper and NM carry handle that have a 1/8 match SS barrel, FF handguards, and a 1/4 X 1/4 MOA rear sight with enough elevation to reach 600 yards?
Just because your local matches are at 200 yards, that doesn't make them easier. The targets used for prone rapid fire and prone slow fire have scoring rings that are reduced in diameter (scaled down from the 300 and 600 yard targets) and are very challenging. The only difference is that you won't have to worry as much about windage corrections. But even then you still cannot completely ignore the wind, particularly when shooting on the 600 yd target rediced to 200 yds for prone slow fire.
Buy right and cry once.