The short answer to your 'Am I a good candidate?' question is "No, probably not." You can effectively move teeth with Invisalign in some (but not all) directions. You cannot, however, expand bone well with it. And in your case, it looks like constricted arches are a very large part of the problem. Your arches appear to be severely constricted and would need expansion if proper results are to be achieved and maintained over a long period of time. Expansion of the bone cannot be done with Invisalign. Having said that, after expanders, Invisalign could be an option.
This is a valid concern. The bone where the incisors are, in the front of the face in the upper jaw, called the maxilla, meaning that you would have stability in the anterior (or front) of the mouth, and no stability in the posterior (back) of the mouth. This means that the pressure is all being place on the front implants, which also are placed in the pre-maxilla bone, which is the softest bone you have in the body. Without the load being shared, there is a much higher rate of failure in these implants. We can also add bone tissue to the pre-molar or molar areas using a technique called a sinus lift to create enough bone tissue for implants and to give better stability and better retention, as well as better force distribution. Sometimes implants can be placed at the same time as the sinus lift and you can be fitted with a non-removable healing denture, but this is not always the case.
Acrylic is generally cheaper and lighter, but won't last as long as zirconia and will not look as realistic. So, if money is not the driving force in the decision, then Zirconia is the way to go. "Lifetime" is a tough expectation to put on the longevity of any dentistry because materials aren't indestructible. It also depends on the bite that the all-on-4 is set to. If it's a proper, neuromuscular bite, then there's less stress on the arch, but there are also things like your eating habits, proper care of the dentures and implants, regular hygiene appointments, etc that will all affect how long the dentures last. Zirconia ultimately will last longer because it's a stronger material. Also, depending on the restoration such as a denture or bridge, even though the technique is called 'All-On-4', you will likely need more than 4 implants if you opt for Zirconia because it's a heavier material and will require more support.
