Since what's possible or impossible is the only options you've given me, Is it "impossible" for your mom to just do all her banking in person or over the phone? The phone works perfectly fine.
See how that sounds?
Anyways, having a central gatekeeper to all those essential services is not optimal from an economic or technical perspective. (Again, notice I said "not optimal" even though you created the false alternative of "not impossible").
Your concerns about malware are valid. Or at least as much as they are valid on MacOS. Which is to say - somewhat, but not enough in the balance of things.
And yes, I have elderly parents too. I plan to continue educating them.
And your malware claims are total BS There's been alternative stores on Android for years and the malware situation remains stable to slightly improving thanks to hardening of the OS that Google did resulting from the goal of supporting multiple stores. Far more people in the world use Android and bank on it than iOS so the idea that the sky will fall if Apple opens up is pure ridiculousness, utter silliness.