Tell him he can make some gil if he changes nations. Plus it's easier the second and third time around. You don't have to climb the tower again, if you kept your key you just go straight to the basement. Also, the magicite quest is easy too. Just run straight to the magicite points without having to run to get all the key items because you already have them. Lastly, you can solo Shadowlord. I solo'd him as a 72 PLD/RDM. It took near 30 mins because when he does his invincible, the only damage was from my enfire. But he went down fairly easy for all that.
If I remember correctly, there's 4 spots on the wall where you can put up stuff. 2 on each side of the room. I might be wrong. Hopefully, I'll find out tonight. I hit rank 9 last night. As far as the moghancements are concerned, I'm not sure which enhancements they have. I think they are different, so they'd eventually cancel each other out. As far as the bonus is concerned, you only get the bonus from the nation you belong to at that time and doesn't increase your CP any more as far as I know.
A lot of the missions stuff you might be able to solo, depending on your jobs, but it does make it much easier if you have a couple ppl to help you. Basically, you can Trio all of them with 75 Jobs, except maybe 5-1 Skele fight because it is level capped. Unless you can go with 3 SMNs. SMNs pwn that fight. When I did it this last time, we did it with 4. SMN, WHM, WAR, NIN I think the set up was. Blade penalty was negated by Banish and SMN did most of the damage, while the WAR pulled the adds close and the NIN tanked.
Even the BCNM fights can be trio'd as long as you have a sleeper. Either BLM, RDM or BRD for Sleepga. Heck, if done right, they can be solo'd.