The photo paper is divided into three packages. One pack fits into the "loading drawer", which can be removed and closed for transport. You also get 3 colour cartridges, these should be changed together with the paper.
So the answers to your questions would be: Yes, 36 photos.
You can return these like normal ink cartridges to a specialist retailer, e.g. Digitec or Migros customer service. Wherever normal ink cartridges are accepted, these should also be accepted.
There are several packs included. As long as the individual packs have not been opened, the paper will probably last a very long time. In my case, it was still tip-top after over a year.
Yes, the photo paper is compatible with the Canon Selphy CP-1500, it is the RP-108 photo paper in 10 x 15 cm format with 108 sheets, which is especially suitable for Canon Selphy models such as CP1000, CP1200, CP1300 and also CP1500.
Yes, it works with the iPhone. You can download the Canon PRINT application. In my case, the printer doesn't see my wifi modem so I set the printer as a wifi modem and connect my iPhone to it. From then on, it works very well.