In general I don't worry about battery, but I'm very careful
In this particular situation I have removed instrument cluster and disconnected all connections from it without disconnecting battery. I have done this way many times. This time I removed cluster because I also fitted CAN Gateway behind instrument cluster and that project still continues. That project is not related to radio controlled clock. When I connected connectors back in place I don't remember if connector with clock receiver was connected before or after connector that supplies power to the cluster, so I won't bother if I was you.
With cluster out I tried to access the OEM place for clock receiver in the middle of dashboard. In ELSA Audi claims you can get there through cluster opening, but I just don't understand how I can attach bracket with a screw in such tight place. I tried to leave receiver there temporarily without attaching it, but it didn't catch signal after 2 or 3 days and I explained it to myself because I have heated windscreen which blocks all radio waves. I then put receiver into driver's door pocket (it has long wire) and the antenna symbol appeared next morning. I'm still contemplating about best non-OEM place to attach it to in my car.
If you'll decide to pull the cluster just throw towel on steering wheel upper trim / cowl. If you won't do it the cluster will scratch it. It can also scratch the upper area of steering column surround trim, so be careful. When I removed my cluster first time I have wrapped it with Audi's sticky felt tape that partially protects me form that trouble. I still need to be careful. I personally find that removing knee trim is easier.
How it happened that you don't know your RNS-E code? This is weird.
No VAG COM cable? This is even more weird
I can't do anything electrically without VAG COM. I purchased genuine cable recently, but I can sell you my old one. It works with VAG COM v908, which is more than enough for K-line cars. Because I now fit CAN Gateway and experiment with 'over the CAN' diagnostics I need to have latest VCDS so I've got genuine cable. Or just purchase Chinese cable off ebay. It is a must have if you're serious about car.