I strongly disagree here. Unless you are using all minimal templates (which is an advanced feature), at some point you will have not enough memory for more or less normal use. I recommend at least to buy a computer, which you can upgrade later (both Librem Mini and 14 can be upgraded).
According to this list, it’s not that simple at all.
For detailed comparison between Mini and Librem 14, you probably should go to Purism forums. Apart from that, USB keyboard is not recommended: Device handling security | Qubes OS.
You can also make backup to a USB stick or hard drive, so it depends on your needs.
Yes: Device handling security | Qubes OS
If you are asking about storing your passphrase in an offline VM, then read this: Data leaks | Qubes OS.
Any software working on Linux should in general work on Qubes.
Accessing a Puri.sm Librem Key
It depends on your needs. If you only use web browsers, it will be easy. If you need to use VPNs and crypto wallets, it will be harder. You can always ask on this forum.
If it’s not in the HCL, then it’s probably not tested. Mini is there.
Consider a laptop instead to avoid security problems with a USB keyboard as I explained above.
Qubes documentation is generally well written but not always easy to read. I hope someone else can point to a good Linux learning guide.