I'd probably use the robe of resistance if you really have no other source of resistance - with high dodging you shouldn't need too much AC for the early/mid game. Once you can find a way to at least not be cold vulnerable, use the fire dragon armor for the late game. Preferably being able to have one pip of rC even with the FDA is best, but just don't go around vulnerable
I don't have the spreadsheet any longer, but there is one that shows what your spellcasting penalty per armor skill per base -EV is. If someone still has that feel free to correct me, but the rule of thumb was around 9 armor skill for -EV and you have no penalty at all, but for -2 you have to go all the way to 27 to remove it (but it's very tiny at any reasonable armor level, you are really fine around armor 10), and anything -3EV or over will always have a penalty. For -3EV and higher going from 0 armor skill to 27 armor skill cuts the penalty by around 50-70%. So if you have high armor skill casting in fire dragon armor isn't that much of a penalty. There will always be some, but getting the magic school 1-2 extra levels will make up for it.
Yes, 14 skill is min delay for a 130% trident. Trident is not a good end game weapon, but a demon trident is. You just have to find one, and shouldn't have to change your skills at all.
To remove all shield penalties, it's 5 skill per base -ev. Buckler is 1, shield is 3, and large shield is -5. So skill level 5, 15, 25. For large races (centaur, naga, troll, ogre) they only need 3 skill per ev. 10 is fine if you're working on other skills first, the penalties will be small at that point, but in the long run, get 15.
Buffs that generally help melee: Abjuration, stoneskin, regeneration, blink, control teleport, insulation if you need rElec, swiftness, flight. I'm sure I'm missing a couple, but those are the ones I tend to use.
If you meant learning Ozocubu's Armour as the ice armor spell, keep in mind you have to be using light armor (EV penalty of 0 or 1) to cast that, otherwise, use stoneskin. Unfortunately you don't have as high aptitudes for earth, but really I'd focus more on the blink/flight/regen sort of spells instead of an armor spell anyways. You can learn Phase Shift for bonus EV if you're willing to learn a decent level of magic.