I don’t have a second internal drive, so I’ll probably use an external hard drive or cloud storage.
Best to get both. Two copies of the data is better then one. Ideally have three. One being remote is also a plus as that will protect you in case your house burns down or something else drastic.
1.What’s the best way to back up and restore my data safely while preserving all metadata?
There is no real best way. Loads of things can be used to achieve the same purpose. But there are two main end goals: either copy/sync the data with a tool that can preserve timestamps or use a tool that can archive the data into a single file (that can optionally be compressed - like zip). Or for cloud providers they typically have their own tools that can do the syncing for you and will typically preserve timestamps by default. Just find one with a sync tool that also works on Linux.
There are loads of backup tools for windows that can do these things - though you should use one that does not use its own propriety format but instead something more open that can be read from the Linux side. I don't use windows these days so cannot advice on what the best tools here are.
Otherwise you can do this from a Linux Live USB using Linux based tools. If you are fine with the CLI then a simple way is to just mount the drives and use the rsync
cli utility to sync the data you want across. rsync also lets you cancel and resume the sync which can be handy for larger directories. Or use the archiving tool tar
to create a archive of the data on the other drive (this can be optionally compress). You should be able to use the filemanager to create archives as well which might make things a bit easier and should preserve timestamps - but just copying the data in a filemanager I don't think tends to preserve timestamps though so best to use the CLI for that or a dedicated tool.
What ever you decide to do test it on a small folder first and see if it does what you expect/want it to before committing to copying large amounts of data.
3.How can I access or use files that are only supported by Windows once I’ve switched to Linux? Are there workarounds or compatibility layers?
This depends on the file formats you care about. Some things are already supported by native Linux programs, others are not. Some windows only programs can work in compatibility tools like WINE - but I would only do that as a last resort. Personally I would figure this out before you reinstall your system. Start by finding Linux friendly programs for the files you need to use and see if they work with the files you want - if not try converting them before switching. Most Linux programs can also run on Windows so it is best to find these alternative and test them out before wiping your windows install. You can also boot up a Live USB and check things that way before committing to a install and wiping your system.
You can also (in addition to backing things up properly) buy a new SSD for your laptop and swap them around - keeping the original install in place in case there is something you forgot or need to switch back to access some old program you didn't think about.
You should have a live USB of the distro you want to use and ensure you have backups of all the data you care about. Then the easiest/quickest/least error prone way is to just wipe the whole drive and reinstall the distro from the live USB. They typically have an option to wipe and install things from an empty drive. Then just restore your data from your backups.
You could also, after creating backups, from a live USB environment delete the windows partitions and resize the linux ones - being careful not to delete the EFI partition as that is where the boot loader lives. You can optionally delete the windows boot loader from the EFI partition as well. If done right you should still be able to boot into your linux system afterwards though when missing with partitions like this, especially when you don't know what you are doing, it can be easy to break the boot systems. These can be fixed from a live environment and there are many guides out there on how to do that.
You can always just reinstall the system again if you mess things up and cannot figure out how to fix them - so always prep for that case by backing up everything you care about first.