The templates you use should make no difference to your placement in search. Many sellers, including myself, use only one specific template for all of their listings. If eBay thought they were duplicates, at least with FP, they would remove them.
I have one gallery photo but some listings have several in the template. When I launch them from SSB I have the option to add 11 additional ones to the gallery photos. As long as they are the right size they should be able to be seen on mobile devices.
I think eBay would like us to have short descriptions for the mobiles because those buyers probably don't read the entire description anyway. Many buyers don't read the description when they are on their computers. I like to have lots of content in my listings because it can be picked up by search engines. I think I remember reading that the new SE that eBay is using will also pick up information from the description, and not just the title, for searches.
Also we are rated by buyers for our descriptions so I give them as much information as I can.