Shaq is a backup, at best. He'll probably see spot minutes starting if there is an injury but he lacked the ability to feed the ball into Ayton during Summer League, which is a problem. He's only played with 1 or 2 other scorers at a time and has no experience with a team with 3 guys who can knock down 3's and a lowpost option as well. We'll see what he can do but I won't hold my breath. There is a reason he was in the G-League and made his NBA debut last season. He's not a good 3pt shooter, not that good at driving to score, has trouble making entry passes inside, and has only been guarding backups and fringe NBA players.
I would be pleasantly surprised if he can shoot better than 35% from 3 and keeps an assist to turnover ratio better than a 1.5. If he starts, I think he'll struggle to even get 10 points on his own. He won't need to be a scorer but since he's not a good facilitator he needs to give us something on offense but I don't see what that might be right now.
I'd rephrase your opening line: Shaq is at his best as a backup! He's got a good motor but I weriously doubt that he could play starters minutes and still keep up the defensive pressure. He's a lot like Bledsoe at his best, he disrupts the opponents offense and that burns a lot of energy. We never saw Bled at his best once he became a starter.
I do agree that he needs to get better from 3 and in the SL he took some of his open looks from that distance though his first instinct is to drive. He's quite a good finisher on his drives, so ideally he'll choose his spots - shoot the open 3 unless there's a good lane.
I think you took the wrong message from the summer league - true he forced some passes to Ayton that he shouldn't have but at least he was trying his best to feed him. I much prefer a PG overdoing it in that direction than waiting for ideal situations to show up. I have little doubt that Igor can teach him when to hold up the pass - and also get Ayton to work himself into a better position to receive the passes.
Who starts at PG? Okabo. He's not really a rookie. He's been starting in a good pro league and he's not a soft Euro. He's rather hard nosed and keeps his composure. He's a decent defender and has good range on his shot. Reeds D looked good in SL but if it slips, Shaq could step in for him and play alongside Okobo for stretches and have him pressuring the opponents PG.
What about Melton? We'll see. If he's even better than Harrison we are truly stacked at PG.
