M2-700 and M3-700 have the same battery, but the difference is in rock guard and charging port. So it will work as long as you can also swap the rock guard kit (LEVO MODEL 2022 BATTERY ROCK GUARD KIT) ...
This is not exactly the right one for Code RE, but you you can use it anyway if you cut fixed banyo off. For Guide RE you need the same olive connection as on the lever side. So basically you need only bare hose and two olives connections which can be much cheaper than buying this kit with fixed...
If motor is still in under warranty then they will replace whole motor, if it's out of warranty you (i.e. qualified mechanic) can replace the belt only ...
Connecting link its just one (outer) link, not 2. If your chain is too short you may snap the derailleur off when you'll hit some jumps and bottom out the rear shock ... Easiest way to properly size the chain is to use lenght of old one. If old one is not available, see this video:
For seting...
That depends on which cassette do you have on it. If its 11spd then PC-X1 is the right one.
Number of links again depends on rear cassette, chainring and chainstay length. WIth my MY20 Levo with stock groupset chain length is 116 links.
Chain is simply shortened with chain breaker tool.
A new...
Top left on second photo there are brake pads and a chain ...
According to this diagram here:
The bleeding port is here:
And as Mikerb said, this caliper doesn't have bleeding edge port, but regular bleeding port (same as on the lever).
If you are checking chain stretch regularly and not let it go pass 0,5% (for 12spd drivetrain) then it should be no major problem matching new and old components ... I think he should hear the noise (and possibly also feel it while pedaling) if its coming from chainring or cassette ...
This also depends on your country law, in my country warranty gets restarted everytime essential component of the product is replaced, and motor is obviously essential component of the bike ...