Waver would never trust Kirei.
He also likely knew that Kirei was a master.
Iskander really isn't a good servant to have on the 5th HGW.
Mudusa kills him a glance from her mystic eyes.
Saber has an army killing lazer
The only thing Iskander has that could hurt Berserker is Gordius WheelzZand that will woek only once.
If archer plays as an archer he can kill him.
I guess he cam just run Sasaki over til he dies, poor guy.
Medea after she made her temple is started draining mana from the entire city beats Iskander for his lunch money.
Even if Waver hacks the grail to make Gray the one providing the mana it won't make much difference.
His best bet is allying himself with someone. If professor charisma works his magic he can get a team with either Rin and Shirou going, for sure.
Saber wouldn't be happy tho