re: Problems Repair

From what you are saying my guess would be that you may have caused the problem by cleaning up too soon. If it was down flat and you were happy, it was likely a good glue-up. However if you then cleaned the surface with water, even cold water, before the glue was completely dry you could both weaken the glue underneath and cause new expansion in the veneer. This would be a particular problem with a figured veneer. If you had waited until it was good and dry, the iron may have worked better. The only reason for making a slit (I think) would be if the blister was full of glue as would be the case if your glue was too thick / too cool. Your problem was more likely caused by wood expansion over glue that wasn’t completely cured.
Just my opinion. Hammer veneering can be more of an art than a science sometimes. The good thing is you are learning all of this by experiment and not on a piece that needs to be perfect.