1. Aegis boon is great in general, but I'd put it below things like Cure II, Protect, Shell, Shock Spikes, Stoneskin, and some sort of nuke if you're soloing CON. I'm thinking it just won't proc very often unless you're GLA. Note that when it procs it blocks all of the damage from the attack, so even if it only procs a couple of times over the 20-30s, it's still helpful.

2. As of 1.18, guild marks are tied to the city where the guild is, so you'll get CON guild marks on Gridanian battlecraft leves R20 and up for the time being. Naturally when your CON is R30 or R35 you'll stop getting offered CON marks on R20 leves.

3. I believe you get a substantial boost to SP below R21. The best thing to do now is to either level another class so that you'll be able to solo mobs 10 ranks higher with relative ease, or find people to grind mobs with. Finding a Linkshell if you haven't already, especially if it's a large and active one, can help with the latter.

4. On your weapon I think that determines how the damage from Spirit Dart, Elemental Shroud, and Nature's Fury is expressed, e.g. if you equip a [wood_type_here] Wand of Flames (100% Fire) your regular attacks will do extra damage to enemies that are weak to Fire.

5. They probably aren't worth it against those enemies. The DoTs can be very useful against bosses and NMs, which don't take much damage from nukes unless you've pumped INT quite a bit. I would get THM to 4 or whatever it is and get Blood Rite. Learn the elemental weakness of the mobs you fight through experimentation, and use Nuke -> Nuke II -> Nuke when you learn the tier II nukes at...26, I think. Until you get the tier II nukes, mix in a tier I of another element instead of a tier II in the middle. Use Blood Rite and Profundity when you can, and I recommend using Profundity on the tier I because the cooldown of the tier II is long enough already. My damage routine looks roughly like the following (for Fire):
Blood Rite -> Fire -> Fire II -> Fire -> Profundity -> Fire -> Fire II -> [probably need to heal, throw Elemental Shroud, etc.] -> [repeat]. Obviously it depends greatly on whether you're soloing or in a party, how much damage you're taking, etc. If your fights are lasting long enough, that you're not being slowed down by it, using Spiritbind is a great idea, especially with the increased cost of Cure spells.

Thanks for organizing your questions. It made them easier to answer. Good luck out there!