I agree

... and pay 5-10 $ fee per transaction? I doubt it.

Right now the median fee on lightning is $0.000226047.

https://1ml.com/statistics

Next question: where can I get Lightning banknotes for offline payments?

You say that like any argentinian would walk around with money or any valuable after he abolishes the police.

You are aware people have been carried valuable items with them long before the invention of police?

Lightning is not a solution, as it requires buying into a channel and not getting your coins until the channel closes.

You can always use level 2s for small transcations.

Sounds like a fun time waiting for transaction to process in the shop

And potentially wait many multiples of 10 minutes if the mempool is full, making it potentially useless.

Lightning doesn't require waiting. Block time is not relevant unless you're sending a very large amount of money.

I would say Tether based on the exchanges.

The reality in Argentina shows that exchange houses prefer Tether over Tron. This is not an endorsement to USDT, just the reality. They will look at cryptocurrencies with high liquidity and low fees.