1. That's largely going to depend on what set bonus you're after. Many of the best defense bonuses come from six slotting a set, and recharge is typically at five slots.
2. In general I try to get the damage to about 95%, give or take, and then prioritize other stats depending on what I need. For example a nuke I'm going to prioritize recharge over endurance, but a tier 1 attack I will likely do the opposite.
3. I'd frakenslot for now, and replace the "lesser" IOs as you acquire your final stuff. Things like Miracle +Recovery and Numina unique for Health are good ones to get early, as is the Scrapper ATO proc that gives you +50% Crit chance when it goes off. I'm also a big fan of putting the Guassian's chance for build up proc in your Build Up power. I've never played Dark Armor, but I do know you have a ton of toggles, so Miracle and Numina are good ones to start with. You can get them both for about 8 million combined if you're just a little patient.
4. No, you're good.
5. My guess is the person likely did it to squeeze out the final bit of defense needed to hit the soft cap. Taking both is not typical for end game builds looking to min/max.