Cross Arch stabilization for Adult Cleft Palate patient?

I am not sure there is a right or wrong answer in this case. I feel you have to plan the treatment to the best of your ability and taking all factors into consideration. Then get patient acceptance and proceed. I think you have carried out the surgery well, and have conceived a satisfactory treatment plan. Your final consideration is whether to proceed with effectively a “fixed-fixed” bridge across the bony defect or treat with essentially a “fixed-free” plan. I feel that how you finalise the treatment from here will need to be results based. I doubt there is much evidence as to what is right or wrong because it depends on the individual circumstance. In your patient’s case, could I suggest that you aim to do a single bar over denture and see what happens. I suspect it will be very stable and most satisfactory. However if there are problems, you could modify the bar (ie section it) and still use the same denture (probably without any modification). Therefore, in your case, I think you could proceed with one treatment plan in mind but if required, change the plan by make a relatively minor adjustment but still using the same componentry.

You can stabilize across the arch, as long as the implants stable , I wouldn’t do the same in the mandible. No flexture in the maxilla happens only in the mandible

If possible I would place an other implant in the right maxillary canine position. Once it integrates I would do a Fixed Full Arch Porcelain to Metal Bridge and have it cemented to the Abutments.