I don`t have a BV3, but it should be always the same with (older) Beovisions:
1- Afaik it`s not possible to change single PUCs, as you have to change an eprom in older BVs, that includes a complete set of devices.
In newer BVs (BV7 - 10 ...) your dealer can arrange a PUC list on his computer and transfer this list to your Beovision.
But you can control your DVD player via a Lintronic box, connected to the Scart lead and hidden with the DVD player.
www.Lintronic.dk
Or you get a special Scart lead from oneremote.
www.oneremote.dk
Or let Ulf modify your box:
www.ulf-kiener.de
2 - Yes. Your STB will be controled via a Beo IR emitter connected to the front of the STB. Doesn`t matter, whether your stb is connected via RF,
Scart or HDMI...
Stefan