The HP Boost does not stack with the medic-gun.
It heals for 60 HP, and bumps the max HP by 50. At the end of the 30 seconds, the extra 50 HP disappear immediately. If you are above 300 HP at the time, you are reduced to 300 HP (assuming no over-heal). Otherwise, there is no change for your current HP.
If you are currently buffed to 350 by the bar, a health-pack or resupply cabinet will heal you up to 350 HP. However, it appears that dispensers will only heal you back to the original 300.
There is more detail at the official wiki.
The compression blast range appears to be slightly shorter than the range of your actual flamethrower, from personal experience. You can compression them anywhere within that range and you'll reflect. The angle that it gets reflected back at is dependent on where your cross-hairs are aimed when you hit the compression blast.
Additionally, for grenades, there appears to be some determining factor with return velocity depending on your distance from them. I can't find a lot of info about that though.
Rockets will travel indefinitely until they hit an object. Arrows seem to function similarly to grenades, but less noticeably so due to their natural high speed and lack of "bounce".
In regards to lag compensation, the compression blast is NOT lag compensated. You're going to have a harder time reflecting rockets / grenades / arrows if your lag is higher, especially when dealing with the higher velocity ones like the Direct Hit rockets and arrows.
Edit: As per the link in the comments, edited the above to indicate that the compression blast is NOT lag compensated.
It's hard to say "when" you should compression blast. It's a function of distance and velocity, rather than time from fire. With any projectile, it's nearly impossible to reflect at short range without some sort of luck / godly reaction speed. At longer distances, the Direct Hit and arrows can still be relatively difficult to reflect. Again, it's about the distance, not the time before it hits. The best way to get a feel for it is to play it and learn it intrinsically, not go by a paradigm that there's a hard rule for timing it based on seconds.
Best Answer
Yes, but only in specific cases.
The enemy's 'Capture Progress' bar will only deteriorate faster if the point is currently in a state where it can be captured by your team (in other words, only 'Neutral' points). If you are standing on a point your team owns that the enemy tried (and failed) to capture, you will not increase the decay of their capture attempt:
In other words, to capture a neutral point, you need to 'undo' the enemy team's progress bar before your own team's progress will start.