It seems that everything that your dentist told you makes sense. The only thing that you didn't mention is whether or not you will need to redo the inlays and onlays that you just had done. The answer to this is that as long as the access to the nerve chamber can be confined to the gold restorations placed, a simple filling can be placed once the root canal is complete. This technique would not require remaking the restoration. In the event that the integrity of the restoration is compromised to the point that it is separated into two halves, it would be best to remake the restoration. It is unfortunate that this happened, but it is totally repairable. The end result should be predictable and long-lasting.
This is a good question and one that I talk about all the time in my office. I am glad to hear that your mom is having an onlay done. What this means to me is that her dentist is conservative and is looking to preserve as much tooth as possible in restoring her teeth. Both of the materials he or she has suggested have their benefits and disadvantages. The porcelain one is very natural-looking, blending in with the other teeth to the point that you can't even see it. The gold one, while not as esthetic, will probably last a lot longer than the porcelain. The longevity of the restoration has to do with the materials and the cements that we use. I will usually explain the pros and cons to the patient and then, together, come to a decision. A good rule of thumb is if the restoration is in a highly cosmetic area you can use the porcelain one. If it isn't, use the gold one. It really comes down to how cosmetically conscious your mom is. Overall, my treatment of choice would be the gold onlay. I hope this information helps you and your mom.