You're on the right track. I like to beef up the floor system then use level quick as my tile base. It comes out perfectly flat and level, so the tile is easy to set. With electric radiant heat in the floor I always install a second temperature sensor, as those tend to be the part of the system that fails. Your spare sensor is already there should the first go bad. Also note that most warm floor systems aren't heating systems, they just keep the floor warm, so you still may need a primary heat source.
Damn! Where were you when I did my electric radiant floor heating? Well, fingers crossed that my temperature sensor never fails - that's a great idea, given that they can fail - and it would have been trivial to install a second backup sensor in case the first one ever fails.
Aren't a good number of those cuts against international building code? I'm certainly no expert on the code, but I have done a little reading when looking how to do wiring properly. However, I recall seeing that you're only allowed to cut so much out of the board, you aren't supposed to cut a certain distance from the sides of the board, and you aren't supposed to cut close to the top or bottom edges. I'm pretty sure I see cuts in those photos that go against all three of those.
Don't quote me, and this may only apply to vertical members, but I think it's 60%, and 40% for load bearing.
Long-term, it's a risky decision not to get proper inspections. I suppose every do-it-yourselfer considers themselves above such things, just like we are all great drivers, and it's the always the other guy that is the problem.
To an extent, I agree. But, I don't think that's true of every person who does their own work. Right in the OP, he has talked to some of his construction buddies, and understands sistering joists - but is asking for 2nd opinions. And, I think he got a really good 2nd opinion with the 3/4" plywood option. And, I don't think the OP sounds like someone who just purchased his first hammer and is searching for a project to tackle.