"Could Christ have prevented his alleged crucifixion?"
Generally, this type of question, "could someone have done other than what they did?" ultimately means, "was there a point at which the singular cause of the event was the will of the person involved?"
If I have correctly understood the intent of the questioner, the answer is absolutely (John 10:18).
"If he could have but didn't, wasn't that suicide?"
If "suicide" can be properly defined as "not preventing your own death", then certainly. If it can not be properly defined like that, then no.