Morning! Busy day today, as I have to hunt for food! Don't laugh, those TV Dinners are hard to hunt!
Just as a heads up, I will be writing up the rules as I have time. But these are some good question. In order.
1 - There is no limit to the floors, good catch! And while almost all dungeons in the NDDA are below ground (The Revival and the Streetbrewers are exceptions), there is no rule saying it must be. You could make a tower, should you so desire. Or a clock tower, since you are in London.
2 - They have their own size limits. If a corridor is past 10 feet, it counts as its own room. However, non-room corridors are suppose to be "safe spaces" to start off with. This does not apply if they try and turtle up in said corridor
3 - Room size is unlimited per se, however building bigger rooms takes up more mana, and there is a mana cap for what you can use in a Season, and in a particular Dive.
4 - One boss per room. However, multiple people, creatures, or things could count as "One Boss". That's admittedly up to League Discretion on each circumstance.
5 - If the caster can summon it, and it won't break the dungeon, it's generally allowed. However, Summoning without items burns particular stats depending on the type, strength, and amount of summons. For a Boss Vs. Boss situation? It'd depend. And there is a cap on how much mana the Dungeon can use to improve the Offense (The Offensive Cap is separate from the Defensive Cap), so the Dungeon can't just boost someone's Soul or Mind to crazy degrees without shortchanging someone.
6 - There is no limit, other then mana. It does generate mana to create traps and challenges, depending on the situation.