Stablecoins are not an alternative for Bitcoin, it is like you say ice cream is an alternative to healthy salad because both can be eaten. Sounds correct but it is completely wrong. A stablecoin can not be an alternative to Bitcoin, it can be used for some similar things like making payments but you can make payments with all kinds of things. Bitcoin's primary utility is not fulfilling just any kind of payment functionality, it is fulfilling a peer to peer, censorship resistant payment functionality. Stablecoins do not offer that, they can be frozen any time by the issuer. You can be put under sanctions and many other things.

You are using the wrong numbers in your theoretical considerations. The maximum number of people to migrate en mass to LN is capped at the current maximum active Bitcoin users, and not even all the users but only those that are using their own wallets and actually transacting. Then you add in a factor for migration because 100% does not make sense, who would use on chain Bitcoin then? So yes, if you arrive at a realistic number of users they can be all migrated to Lightning. They don't have to be migrated in a month. That is the problem of your theoretical scenarios, they make all kinds of wrong assumptions and you can make a problem out of nothing. Look at problem, we can't migrate 10M users in a single block! 

That data does not show anything. That is not how you conduct science. Those transactions are not transactions that would be done in Bitcoin otherwise. They do not provide any features that Bitcoin transactions provide.

No. They could be replacing it and you could still see an increase in card payments at the same time.

Bitcoin did kill Western Union for anyone who knows how to kill Bitcoin. The remaining users are at their own fault, we can't force them to adopt the better option.

Bitcoin has already been doing that, and there will be more patronage over time. However, it will not take the place of fiat, regardless of how we want it to do it, as fiat has some preference and qualities that Bitcoin doesn't have.

What you are inquiring about will cause issues supposed Bitcoin is the primary means of exchange. There will be a huge congestion, high fees, slow transaction completion, among others. Bearing the burden with other cryptos and fiat as we see now is the best approach.