It is detectable once it happened. If you run your own node, you can tell when it is "reorganizing", i.e.: discarding one branch for another. One block reorgs, or two block reorgs are somewhat common (citation needed, I don't have the numbers at hand). That's why you see most merchants considering your Bitcoin "paid" only after six confirmations. They are protecting themselves against a Bitcoin reorganization of more than six blocks. The more money you accept, the longer your confirmation period needs to be. Some protocols require 100 confirmations.
What happens to Bitcoin when this happens: to "Bitcoin" the software, nothing. Everything just keeps going. To "Bitcoin" as a currency, your guess is as good as mine. It depends on what the "social layer" (the community - devs/users/stakeholders) decide is the best course of action. This would be considered an emergency event.
EDIT: You might be wondering why it is not detectable before it happens. It isn't detectable before because the attack would be privately mining their fork. That is: they would not broadcast the evil blocks.
You are misunderstanding the attack. At 50+% of the hashing power, you are guaranteed to win a race against the honest block producers. The attack is a double spend: you spend a coin C to pay ADDR_A while mining a private chain that spends the coin C to pay ADDR_B (you can, of course, be ADDR_B).
One ADDR_A accepts your payment, you post your private chain publicly.
You coin C cannot be spent to ADDR_A and ADDR_B, so the chain must choose which one is it. Because you have 50%+ of the hashing power, your private chain necessarily has more work (generally simplified to "is longer") than the public chain.
You've now successfully double spent.
In fact: you don't even need 50+% to attempt the attack. I did some math recently I believe something like 40% gets you 75% chance of successfully executing the attack over a 10-block-period. The Bitcoin paper has the exact algorithm to calculate this, it is a random walk.
I created an account just to answer this question, seems it seems like a lot of people in the first world can't even phantom the need for people to use an unregulated currency.
Source: I'm from Argentina. I can't speak for the situation in Lebanon or any other place, but I imagine there are similarities. I know there are similarities with Venezuela and (I believe) Iran.
Long story short: when your government deems illegal or at least suspicious for you to save in USD/EUR, Payoneer/Paypal/etc are risky to use. You have several problems:
1. You can't easily use the money on (say) Payoneer. You need a credit card. Credit cards are easy to tract. You'll quickly have someone asking where the money came from. I understand this seems like a good idea in a sane country, but it is not desirable when the only way for you to save is to bend the rules somewhat.
2. You can't put your money into Payoneer/Paypal/etc because, remember, you don't have any USD. You can't buy them. So the only way these are a fix is for people who work for USA/Europe and can maybe get paid directly there.
3. Cryptocurrency is not yet as regulated nor it is easily regulated. Payoneer has to comply with Argentinian law because they are a financial entity. Cryptos are still young and are (at least in Argentina) still somehow being able to sell USD-like assets without selling USD. Example: in Argentina you can buy up to 200 USD per month (well, the lucky ones get 200 USD... won't get into details), but you can buy as many USDC as you want.
4. The _your keys, your money_ moto may be a meme for the YCombinator crowd, but it is not for many people. Payoneer/Paypal and banks *will* ask for documents baking up your funds. Again, this is a good idea if you live in a sane country, but many people do not. You can't just go to the bank and tell them: no, I don't have these dollars declared anywhere because they are illegal to own in my country. I've seen colleague's accounts getting closed due to "suspicious activity" and their assets are frozen. The Argentinian answer is generally: be grateful you are earning in USD, look for the next Payoneer (I think we are using a lot of Dukascopy lately) _or_ switch to crypto.
Anyway. Make of crypto what you will, but people in high-inflation countries with an excess of love for regulation intuitively understand their appeal.
What happens to Bitcoin when this happens: to "Bitcoin" the software, nothing. Everything just keeps going. To "Bitcoin" as a currency, your guess is as good as mine. It depends on what the "social layer" (the community - devs/users/stakeholders) decide is the best course of action. This would be considered an emergency event.
EDIT: You might be wondering why it is not detectable before it happens. It isn't detectable before because the attack would be privately mining their fork. That is: they would not broadcast the evil blocks.