Do you have Jasmine or Melusine? If Melusine, who exactly is in your party? This will affect your strategy a lot, so it's not possible to give good advice not knowing that.
I'll start with your question 5.
Yes, definitely, without any hesitation
turn round and go back. Here is an extract from my notes to myself.
97. Although this is optional, I strongly recommend that you do it and that you do it now. After the entrance you will have to fight your way up the Tower. If you go back down the stairs you are instantly at the ground level again and the enemies will regenerate – so a useful place to do some levelling up if needed and to try and harvest Heaven Jars, though they are not plentiful. They only regenerate until you have completed the Tower.
98. The Defenders of Lo are weak to status effects (except charm), but particularly the one included in their names, so Silent Defender of Lo is vulnerable to silence and so on. Teksab’s Rope of Pain will inflict paralysis about 80% of the time; Clea casts Cat-A-Strophe if she has it, if not, then cast Detract. Starting on level 7, they appear in groups. Levels 7 and 8 are 2, level 9 is 3; after that there are multiple Defenders, but they only appear one at a time until level 14 when you get one, then 2 together and then a group of 3.
99. On level 15 speak to the woman and answer ‘Yes’ and Princes Lo joins your party. The purple crystal behind her has a Magic Trinket. There is a chest with 1666g. Exit and return to Nivkaden to rest and save. Speak to Crystal for quest complete (Q22) and a Magic Trinket as a reward. Then choose to travel to Greenland.
Alex gets level 2 of her fire spell around 48, don't remember exactly.