Yes, it is a legal restriction, because that’s what Congress decided. This is why opening an HSA early in life is a good idea, even if you later switch to a traditional plan.
It is up to you to maintain adequate records, not your custodian. They may nudge you into uploading and store a receipt for your own convenience, but it’s not their job to police your distributions.
Yes, the general consensus is that a digitized copy of a bill is adequate proof of an expense.