I've gotten used to random charges (and always being able to measure) in Warhammer Fantasy, so I'd love to see something similar in Mordheim. I rather like your approach to it. Only thing you'd need to iron out is how Sprint works, likely either 2d6 pick the highest + 2x Movement or 3d6 pick the two highest + Movement (the latter would make sprint less reliable and powerful for already fast warbands like Skaven, which is probably a feature rather than a bug).
Not a huge fan of the rest of your change suggestions though. Setting I penalties and bonuses would put too much emphasis on the I characteristic, and make other sources of ASF too powerful. I agree with Thespian though that ASF needs to be less layered/confusing, although I'd probably go for a very simple route there. Something along the lines of:
- Charging, spears, that one skill, and the one Orc spell, plus anything else I forgot: grants ASF (always strikes first).
- Standing up from knocked down, two-handed weapons, plus anything else I forgot: grants ASL (always strikes last).
- If two models in a combat both have ASF or ASL, they cancel out and strike at I order [ex. a knocked down model stands up while fighting against a model with a two-handed weapon. Since they both have ASL, they strike in I order].
- If a model has both ASF and ASL they cancel out and the model strikes in I order [ex. a model with a two-handed weapon strikes in I order when it charges].
- Multiple instances of ASF or ASL can stack with each other [ex. a model with a two-handed weapon stands up from knocked down, so they have two instances of ASL, meaning they will strike last against another model with a two-handed weapon].
This would certainly shift the balances between two-handed weapons, spears, and other weapons, but dear dark gods would it be simpler than remembering which ASFs and ASLs override each other. (as a side note, I'd also likey want to change spears to straight always grant ASF, not just in the first round of combat)