no you cannot change your island location, if you reset your island gets deleted and a new one is being created at the same coordinates if I'm not mistaken.
Yes, if you restart, your island will be reset at the same coordinates. The only way to get an island next to your alt is to buy the island next to it. This could be difficult, however, as some islands are abandoned and hasn’t been used in over a year. There are some discussions of getting rid of islands that have not been used in over a year like on classic so hopefully this will come in effect on Skyblock.
There's no current way, you'll have to get lucky when creating an island and hope that your alt's island is created directly next to it, or that there is an active neighbour willing to sell their island to you.
You can change your location by joining a coop. After some time, someone else will be assigned your old location at which point you will be assigned a new island when you leave the coop and create a new island.
If you and your alt want islands together, you have to both start over. You will need to coop with someone else until someone takes your islands. Then create new islands for both alts at pretty much the same time.
