1) If i'm not mistaken both dimensional and galatic are in the same pack. It's similiar to how the fairy pack used to contain both normal and dark fairies.
2)interesting observation but i've never really played with the galactic set sadly since I wanted a more straight forward play style.
3)final image will unlock at 2000-2400 height. odds are you had too big a jump in height causing you to skip over earlier images and the gallery unfortunately does nothing to communicate which stages have been skipped over. If donating height through the statue I recommend 150 at a time or something close to it to help prevent skipping through stages.
4) Titania at arena inside night time sells some very special stuff. 3 out of 4 of the things are very useful stuff. The HP Booster gives you 5 max HP permenantly every time you go to sleep. If the HP booster isn't enough for your purposes then win 3 games at the casino so that you visit the VIP lounge. The VIP lounge sells a comsumable HP booster that grants permenant 10 max HP for each one comsumed.
4b) if you think the first enemy in succubus quest hit hard the final boss the quest could one shot valeria if it decides to bring out a certain attack. first time I attempted that particular boss I got WTFBOOMED by a 4k damage hit on the first turn. 2nd time I attempted the boss I luckily didn't see the attack again over a dozenish turns.
5) you need to donate thousands of height for each stage of growth on the statue. You'll need to donate away on order of tens of thousands of height in order to trigger the major event.