2a) My guess would be depending on wave level and gold cost of the tower. 2k gold towers seem to appear around ~40-50 4k gold towers at around ~80
1) The cube level doesn't matter for the drop calculation. I use the same calculation like for dropping regular gear after the rarity is determined. The formula depends on the current wave. Every item has an iLvl at which round it is possible to drop. 2) First, the rarity is calculated depending on the current round. The higher the round, the higher the likelyhood to find better rarities. If the rarity is determined a random tower from all possible towers of that rarity is picked. Again, every tower has an iLvl telling the earliest round this tower can drop. If you roll a rarity with no possible towers at the current round, the next lowest rarity is picked and the same done again until a tower is found.