When a divorce is taking place, if the husband in question doesn’t get along or doesn’t like one of the witnesses who was present at the time the divorce was read, is that divorce valid?

Also, once a husband has given instruction to read or give the divorce, can he later retract it after the divorce has been read, and then seek for the divorce to be a khula divorce instead?

Only if they can prove that the witness is not just. They have to make a case with a religious authority

If the divorce has taken place, he cannot retract it if it’s khuli, but if it’s the revocable one then yes within the idda 3 cycles he can go back to his wife