2. Don't try upgrading your laptop's CPU. You'll probably find yourself with more issues than you can keep track of and more thermal throttling than you know what to do with. It's just not a good idea.
You can't just switch to any gen when they are different sockets plus bios may not be compatible even if they are the same socket. This pretty much limits you to the same gen because they don't update laptop bios. Besides the other reasons, you're better off finding a laptop with a cpu you want and not worry about upgrading. If you want something upgradable, then you want a desktop.
But to answer the question; 5th gen. You still find soldered with 5th gen like any before it.