(1.1) Re; Improvised Actions (and again it's GM discretion so if you feel it adds some texture to your game to not run them like this go for it) - For me it feels like a more than worthy trade-off.
Basic ATKs are a keyworded ability that Generate Synergy for yourself and allies that can be proced by or help proc conditions on Stratagems and Parts (also to my memory each of them has an Element allowing for even more combo potential) - The "cost" Players pay in terms of Credits to get a Part usually gives them more than just a Basic ATK too.
Meanwhile for an Improvised Action a Player has to describe and set a whole scene that's interesting or convincing enough for me the GM to start handing them comparable DMG (and the GM would have to be in an extra generous mood to pepper in some SYN for the teammates) that still doesn't interact with the system as deeply as Basic ATKs - In all my years of running Improv Actions as described Improv Actions have almost always been used as a last resort when either a Basic ATK would be impossible (usually do to Part Breaks) or when a specific Effect needs to get slapped on that no one has a Stratagem up for. ((Or when something like "Scoop Up the Tank" or "Weld Myself to the Enemy" gets brought up that I'm like "OK this falls firmly in the "Other" Category.)) I also say this as someone who did a whole "hits and kicks" Basic ATK build on the other side of the table because there are some nasty part Combos between extra dice, switching elements, comboing into Stratagems etc - I love Basic ATKs and I would honestly be bewildered by anyone who thought an Improv Action was giving them nearly as much DMG (not to mention SYN) as I was shelling out with those builds.
If given all this a Player still felt they didn't want to use a Basic ATK vs just continuing to describe unique Actions for less combo-y DMG and no SYN to themselves, I'd honestly let them just keep using Improv Actions because it wouldn't meaningfully harm any balance I'm going for and would likely just lead to a lot of flavorful descriptions from that Player. If that Player ever complained they felt their Improv Actions weren't doing enough, I'd remind them an alternative utility exists :p And if any Player felt legitimately cheated by another player dolling out basic roll DMG for describing stuff I'd probably have a sit down with them about what they're actually looking to get out of the game and why they feel someone mildly succeeding via creativity is hurting their game experience and we'd try to figure something out.
(1.2) Oh I get ya! I mean yeah Stat Scaling just /is/ just how most Stratagems with Rolls work - A flat non-rolled bonus could be interesting but yeah that's less the flavor I'm going for because it'd probably be hell to balance appropriately. ((Unless different Parts gave you Hull based off different Stats which could be interesting, but also at that point every MEC is just operating the same and only nominally different which I'm already worried the game has too much of))
(3) Yeyeyey ~ It's different strokes for different folks - I think a LOT of Steel Hearts' mechanics (intentionally) grind up against a more BattleTechy idea of system balance and equilibrium which is absolutely not going to be everyone's flavor.