Prius style would be about as simple as you can get mechanically with only the differential as the big new part and no clutches/brakes to worry about but even there you're adding some not so great failure modes, if you lose your engine with locked crankshaft now you can only use your MG2, so limited power, and worse if you lose MG2 (either through dead motor, dead inverter, dead battery) and it's freewheeling, now neither MG1 or the engine can deliver power to the prop, your transmission is just spinning, unless you add complexity with dropping in an emergency clutch or brake too. Certified pilots already get worried when the only complexity you're doing is a single reduction gear on a Rotax etc, adding more and more parts is going to be a hard sell. It also does still weight quite a bit, around 100kg for your differential + motors package, then add on your electrics and batteries.

Biggest alternative thing you could do with your ~100kg budget for a get out of jail alive when your single engine becomes a zero engine is an airframe parachute, they've been pretty damn effective at making a lot of situations survivable, and not just engine failures.