1) it can be worth it, but you shouldn't go further. OD 12 would stop most outside PFs from shooting jumpers with any reliability. He'd probably get quite some steals, too, since there aren't many PFs with Handling or Passing good enough to get a pass off when covered by him. On the other hand, more than 12 would be overkill in most cases.
2) I don't know too much here, but my 21yo C does still experience regular pops, so it's probably closer to 3 weeks.
3) depends on the position where you give him the training. If JS and Driving are trained at the Forward positions, it might be worth it to train them earlier, since they help raising the IS which helps with the elastic training aspect of training ID. 6 pops in ID and 6 in Rebounding in 30 weeks for a 23yo sounds doable, because they do help each other, but it'll be hard. rather expect 4 or 5 pops in each - but I might be wrong.