The 6 and 6s have the same battery life but the 6s just has a battery that is a little smaller so it would be better to get a battery made for the phone you have. Plus the cables have different flex connectors.
The simple answer - You should have not have updated to Mojave, there is a reason Apple does not include them on the update list. What you can do - Find a way to uninstall Mojave back to whatever OS you were running before. Can’t find out how to Uninstall?- Use an external speaker, it is the best you can do.
From what you are saying, it is most likely that the card reader is fried from heavy use by the previous owner(s). You generally cannot fix it separately. The best option would be to buy a new motherboard off ebay. They typically cost from 10-30 dollars used. When you are looking, look at the pictures of the listings with the left Styrophome cover is not damaged. This is the piece that will detect a card has been inserted. If you don’t think getting a new motherboard is worth it, look at your old one and see if any wires are frayed on the left hand side. Good Luck and tell me how it goes!!!
First of all, the Iphone SE is a phone that was made 5 years after the iphone 4s. Iphone 4s was just not made to support iphone 4s with the provider that you stated. 4g service was released in 2010 by 1 carrier- Sprint. It took a few years for other companies to catch on. Long answer short, the iphone 4s Was Not made to support 4g. Good Luck!
One simple answer, apple did not configure their products to run Linux too. If you teardown a pc made to run Linux and an apple pc, you will find the basic parts the same but many supplementary parts too that the PC made to run Linux needed that the apple pc had.