I've tried the iron on method with unbacked veneer a couple of times, and it always splits. In some cases you can cut slivers of veneer to fill cracks, but sometimes not. I have a pair of small speakers with an olive burl that simply has lots of cracks beneath a heavy poly finish. It looks ok, but clearly not pro. I have a pair of GR-Research AV-3s that I hot pressed with plan oak that I finally removed- splits and bubbles. You can try contact cement, but be aware that finishing out the veneer can penetrate and cause the cement to release. The cold press method that you tried already is probably the best option. Raw veneer is simply a PITA.