For instance, does a carbine length gas system have more back blast/gassy face than, say a mid-length or rifle length system?

Do "piston" guns have less (these seem to have settings, like my FS2000 and SIG556 or Daewoo DR200).

Yeah the lower the gas port pressure and the smaller the port size the less gas comes back to the carrier. Other things factor in as to how much gas is delivered and where it's vented once it gets into the action, like dwell time (effected by reciprocating weight).

Piston guns all vent up front. I'm not aware of any that carry gas back to the receiver. There is some amount of gas venting back through the barrel/chamber from residual pressure in the suppressor though on all semi auto firearms.

I don't notice an objectionable amount of gas in my face with any of my rifles and they are all direct impingement. I get more gas in my face with my AK.

You're getting more gas out of your chamber than the bolt carrier.

Which is why a bolt carrier that moves slower is a good thing.

You get gas from two sources: the gas tube and the chamber.  The gas from the gas tube blows straight down the charging handle channel and into your eyes.  More gas from the chamber, yes, but it is less directional.  You can ELIMINATE the gas from the tube by using a gas system, but not all gas systems are created equal.  Those that vent to the atmosphere will still give off a pop, though they won't shoot that gas directly into your face like a DI gun.

Lots of ways to control gas.  Adjustable gas blocks, adjustable gas keys/carriers, forward assist replacements that vent gas out the side, gas buster charging handles, the "RTV Trick".  If you do all of these things, you don't need a piston gun.  I do most of these things AND I have a piston gun... still get a bit of gas.

For suppressed ar15's in 5.56 (mid length BCM in my example case) I've been VERY pleased with a gemtech suppressed bolt carrier.  Huge reduction in blow back/build up vs a regular carrier in a BCM upper.

F-2000 blowback isn't terrible but its noticeable and will really start smoking from the rear cover after a few rounds.  The suppressed fs also tends to throw empties out the front tube more violently if that's a concern.  It has a regular and adverse setting only, maybe you could get a gas plug insert from HDD?

Scar's of course have suppressed settings that help tremendously.