Might come back at this later

.
I didn't have enough time earlier today.
We can debate the details.
Agreed, the kingdom set up and rules and condition for kingdom creation can be debatable. Though personally I don't like too narrow cultural condition; IMHO for instance if a duke of Bohemia & Moravia, he should still be able to become king of Bohemia, but only an emperor, who also is duke of Bohemia & Moravia should be able to create the kingdom of Bohemia. Then again, debatable
I don't like titular titles for the same very reason Paradox doesn't (they can be problematic gameplay-wise).
In some cases I tend to prefer titular kingdom, for instance 'resurrected' old kingdoms like Frisia or the former Carolingian sub-kingdoms like Bavaria* and Aquitania. (*=if I would include Bavaria) I used to think the same over small kingdoms like Galicia, Cyprus and Navarra, but at least Navarra IMHO should be de jure. (However in certain map mods, these can become large enough

.) I guess, agree to disagree?
You see, by yourself you are placing the 'kingship of the Romans' on a different level, see my second post above.
Not really, the nobility of the German kingdom elects the king of the Romans, who also gains the right on the crowns of Italy and Burgundy. This wasn't unique though, for instance during periods where the kingship of Bohemia was elective, only the Bohemian Estates had the right to vote, so not the Moravian (and later also Lusatian and Silesian) estates..
However I do agree, that even as king of the Romans, the king of the Romans was the liege lord of the king of Bohemia. However in the German kingdom (including or and Lotharingia) he also was their king. I guess, that you could say that position of king of the Romans and German king were really merged. (In German and Dutch sometimes referred to as Roman-German king).
Which would lead me to the conclusion, that the (large, is my preference) German kingdom should perhaps be an uncreatable placeholder as long as the HRE exists (or is still ruled by the German king), similar to how the Byzantine kingdom functioned in the Byzantine Empire in early versions of the game.
This will always be debatable.
Agreed, provinces can be tricky; especially with the changes, which happened historically and the fact that some in game provinces can be composed of more than one historic territory or historic territorial developments***, which can affect the set up at various starting points of the game. Another example could be Istria, coastal Istria was a part of Venice, but inland Istria could instead be a part of Carinthia.
(***= like Steiermark, which in 1066 could belong to Carinthia, but at starts from 1192 it could belong to Austria).
You do not need events for what assimilation does already - what we need is to make that one mechanic better.
Oh right

o, I usually set the timer for the standard assimilation much longer than the game. IMHO the current timer is too one dimensional for me; I would have preferred more conditions and also a decision (maybe available after the timer), which starts an assimilation event, where the estates of the duchy and the kingdom can vote on this matter (the estates should like you, if they break traditional feudal bonds). Anyway I forgot this vanilla feature.

o
That would only be the case if Italy were indeed a de jure part of the HRE, though.
I just mean that as a general fix not letting non-feudal rulers be voted for would probably be an improvement.
Besides, if modders wish to have Italy (or other kingdoms) as part of the HRE then this fix would be invaluable. In fact, you could still have Italy as part of the Empire in vanilla if it's conquered and absorbed.
No one would ever vote for an ecclesiastical leader or doge as Holy Roman Empire. It shouldn't be an option.
EDIT: Yes, this is hardcoded. Paradox would need to do this.
I fully agree!