Hi @abcde,
Indeed good questions. I would go with your pharmacist on this, the active ingredients in each would be the same (see here for previous discussions on the matter: https://www.hepbcommunity.org/t/rx-vs-generic-is-one-better-than-the-another/2895; Generic Medication).
I am not sure about med supplies, but the TGA seems to have measures in place for these potential events (https://www.tga.gov.au/safety/shortages/accessing-medicines-during-shortage). Given so many generics companies produces entecavir (as you linked to), it seems unlikely that they’ll all be affected at the same time for an extended period.
I have stayed in Germany for many years, and used the generic form of tenofovir there. It wasn’t really a problem.
Thomas