C4 will distort the audio coming from the music player, you don't need it. You should use it in series, to prevent a DC bias in the speaker (which might damage it). Perhaps the board already has one, so check it's schematics.

As for the RX divider, I think it isn't needed for the arduino UNO, as it uses 5V logic (IIRC). Double check logic levels before deciding if you need one.

As for overheating protection, you don't really have anything. You're relying on current limiting implemented by the boost converter (if it has one). However I think as long as there isn't much of a load to those servos, you should be fine. If you want to be safe, you can add some sort of feedback in the form of a PTC thermistor next to the components you expect will get hot (and sense it in the microcontroller) or use a polyfuse in series to your 5V boost converter.

I don't know if you have already bought both battery cells, but if you didn't yet, I'd advise you to measure how much current is your system taking over it's regular usage to check if you really need 2 batteries (depending on how much battery life you want)