(a)I don't know about the sim card, I pay for my cell service for the iPad.
(b) No, you can't reset the home point without a cellular iPad, but there is a work around by adding a Bad Elf Pro GPS adapter to a wifi only iPad. You will then be able to set a new home point, but you still have to cache the maps ahead of time.
(c)You need wifi to cache the maps ahead of time unless you pay for the cellular service. The GPS works as long as you have the maps cached to where you are flying. I don't like to cache maps, so I pay for the minimum cell service. Most phone providers will let you share your data plan with your iPad for around $10 a month. Some people use their phone as a hotspot by tethering their phone to their iPad and getting their maps that way out in the field. I have an unlimited data plan on my iPhone 6+ with AT&T. Unfortunately, AT&T won't let me share my data plan or let me tether my phone unless I get rid of my unlimited data plan. So I opted to just pay $15 a month for 250 megs of data for my iPad
mini 2, which so far seems to be plenty since I just use the mini iPad for the DJI app.
Like you, I really couldn't find all the answers to all my questions about iPad Mini's, so I decided I had to buy one and figure it out for myself by trying all the different features. I wanted it to work exactly as my iPhone 6+ works, so I pay for the cell service. Most people are perfectly happy with a wifi only iPad and caching the maps ahead of time, and don't really need to set a new home point since they don't move from where they started from. They are mostly interested in having a bigger screen for the camera.