Glad we could help! The only place you'll notice evaporation will be the chamber your return pump feeds from so that's where you'll want your float valve.
To touch on your first issue again briefly. And sorry if you already understand this, but I don't want you flooding your place
With everything off, your sump should be at no more than maximum capacity. If you're at that point, then fire up your pump and you realize you still need a bit more water - you need to reconfigure your system rather than add anymore. Otherwise when you lose power that water will come back regardless of wether it fits in your sump or not
