I prefer not quoting because sometimes is less comfortable. But I'll use your items numbers
1) In the Ultimo 4K I know that there might be either a FBC tuner 8named A) and a Normal one (named B). So, if this is your situation, this might explain your problems (and would make very important to try and configure tuner A for 13E instead of 19,2E).
In my Solo4K I have 2 tuners but both are FBC.
2) Yes: you are absolutely right: my mistake. Obviously, both your LNBs have 4 user bands (I've got also this model) so you have to use in both SCR1 to SCR4. Sorry for the stupid mistake.
3) The discrepancy is not relevant: all Inverto (and not only Unicable/SCR LNBs use the same user bands for the first 4 SCR channels.
Even my new 32 channels Inverto, use the same frequencies for the first 4 channels
1=1210
2=1420
3=1680
4=2040
So, don't worry about the slight name difference.
4) As I said above, just to be sure both your tuners are FBC, try that.
5) Yes: I confirm you won't never be able to decrypt two Tivusat channels (unless FTA, but none of the HD are and even some of the SD, as you said), regardless of their transponders. Is a limit of every smartcams C.I. But....
6) NO: here you are wrong. If you leave the tuners as "FBC automatic"-Inactive, they can't do anything. Actually, the word "Automatic" is a little bit misleading. They will simply be Unconfigured, as they weren't existing.
°°°°
And now another (maybe stupid) doubt: are you sure you connected the right Output to your second SCR LNB: it has got both outputs, one SCR and two Universal.
I say so because I've got 2 Inverto SCRs and they have the same model but... one has got the SCR OUT in the centre of the 3; the other one has got the SCR on the side (the one furthest from the dish), despite the same model name (as they were two different series).
So you have to check not only what type of Tuner is your B but also this fact on the LNB.