There are several strange things here.
> Flagstaff did not have any insulin for me.
This seems… impossible. They could be out of a specific SKU of Lantus pen, but that’s an entirely different thing from a pharmacy not having any long- or intermediate-lasting insulin. They probably had Lantus vials, or other glargine biosimilars, or NPH insulin or dozens of other types of insulin.
> the Tandem tslim X2 requires the user to "prime" the pump tubing with 10 units of insulin every time the pump shuts down or the cartridge is replaced.
This happens with every pump in one way or another to avoid getting air instead of insulin. There is no pump on the market that doesn’t involve wasting insulin somehow (though getting leftover insulin out of Minimed pumps is much easier)
> To tell the truth: I do not even know how to use the pen. I have never needed to do this in my life.
> I've lived with diabetes for 27 years and I've been on pumps for 25.
The pens are just a vial and a syringe combined. If you’ve given yourself injections, using the pen is straightforward.
> Foolishly, I did not bring the backup-of-the-backup. I never have!
For anyone not familiar with diabetes management, these two sentences are the failure that happened here.
Insulin pump failures are very rare, but the only way they can be dangerous is if you choose not to bring your medicine with you. Lantus (the thing OP is prescribed but does not treat as necessary medication) is given to diabetics for this exact situation. The pen is a bit bigger than a Sharpie. One of them can keep you alive for a week or more. There is no valid reason not to keep it with your other insulin.
The lesson here has nothing whatsoever to do with insulin pump brands. OP will never get a pump that can’t fail but thankfully “what to do when a pump fails” has been an elegantly solved problem for many, many years.