Guaranteed gym control?

The limit is 10 berries per Pokemon per trainer, with a 30 min cooldown. So other people can still feed the Pokemon.

No every player have an different count down of 30 min per 10 berry

So nope they can still golden razz

Well given how long the inter-battle animations last, I doubt you could beat a full 6 mon gym 23 times in 30 minutes.

Not to mention, they haven't fixed the gym system to actually keep players from adding mons once the prior ones have been kicked out.

Can we feed a Pokemon when he/she is under attacked?

The one ex raid I was at the Valor team filled the gym 2 over hours before the event, then feed the Mons constantly. The first Blissey had over 148 berries feed and the others behind it were over 100 also don't know the final count. Our team was attacking for the fun of it and there were constant attacks on the gym from both rival teams. I would say your numbers are way low depending on the turn out and you better be ready to empty your bag or have plenty of others helping and only use golden raz's. There were about 100 players or so at the raid and we were all broken out by teams. Good Luck!