I think there might be a way on Roll20, Like you can make characters and pin stats to them and like have their damage show and stuff. Not sure about bonus modifiers or whatnot.

1. My group is post in the IC basically whenever, as opposed to dedicated sessions.
2. Not sure what math you mean exactly, but you can just write down your bonuses next to each stat if you want, and those bonuses only change when the stat's divisible be 5. And as the GM I'll be giving damage reports, so it's a bit more on me.

[s]Advanced section doesn't require systems.[/s] Your system seems decent enough. Like, if it's a simple do you pass or fail thing, then just judge how good the action was. If you want variance, just allow players to assess their stats with, say, a range of minus two to plus two.