Alright, here goes:

1: As is, these guys are the perfect assassins. They have invisible powers that are really hard to pinpoint who they are coming from and can do them at impressive ranges.

I propose they have some visual "tell" about their powers. Add it to Hands that all powers have a visual aspect. Like waves of power rolling from them to the target or the air compressing around them as their power concentrates. This wouldn't be anything that is stoppable, so it isn't like a somatic/verbal component. It is just a thing so that if someone were to observe you use your powers, they could tell you were the one doing it. Maybe not know what it is you are doing, but that you did something.

2a: Force Blast, is it balanced?
I opted to go for the pure ACF, and with my stats I have 27 Hands at level 13 (13+ 12 from Con +2 from LMMH) and a Cha mod of +18. In our game we have chosen to avoid rolling dmg dice as the numbers get absurd and instead do 1/2 +.5 per die (so 2.5 on a d4, 3.5 on a d6, etc) to make the math easier.
I also have the TK Focus tree for Force Blast.

In the encounter, I put up a wall (about 8 Hands)protecting the entire room and preventing a spell from dropping on us and crushing us with snow, since the wall stops things from passing through it (see 2b below for on walls).

While holding the wall, I had to attack someone outside of it. The best option I had was to force blast her. So I threw 10 Hands into it for 325 dmg and a +17 to hit (magically buffed) and still had 9 Hands just in case.

Apparently my group was a little shocked, because they thought I had to drop the wall to do that much damage, and when I replied "no, it could have been higher too if I felt I needed it...but would have almost no chance to hit," their jaws couldn't hang much lower. One said, well it's homebrew so they are never balanced anyway. I said, "it is balanced. I had to drop a 20 to hit with that (I did get the 20, but still)." The fact that my damage potential scales inversely with my attack bonus I feel is balanced because while if I went all out on a force blast I could deal 877, but my to hit would be -21.

They countered with this: if you built an army of these guys, and lined them all up against you, 5% of them would deal that much damage each. Or they could deal 422 with to hits of +7 for even better odds of hitting you as they all attack twice now. With a ridiculous number of saves vs suck. Is that really considered balanced?

I said, yeah. Replace them with random dudes and give them all missile weapons, super ammo, and truestrike and they will deal more (he does this to us, and they do end up dealing stupid amounts of dmg that is just as hard if not harder to live through). He was then willing to accept my claim of balance, but really it was more of a "well, it's homebrew, so they break the game" acceptance.

What do you guys think? Is it balanced? Or am I just deluding myself?

2bA question came up that I had no answer for: does the wall have a mass limit it can support? If a building were falling on you, could you box yourself and be unscathed? I weighed both options and couldn't decide which was more balanced. Does the wall just stop things and any amount is fine since it is a wall similar to Wall of Force in most ways? Or does the wall use some variation of Mental Strength to determine its capacity?