[Gosper]

You can fork endlessly and each fork won't matter one whit because they don't follow the longest chain secured with the most hashing power.

Or your fork can follow the longest chain and abide by the rules so your transactions are accepted by the rest of the network, in which case you're using Bitcoin and haven't forked.

[baby]

The problem is that you never know if you’re on a fork, or if there’s going to be a reorg.

[Gosper]

So you then follow that chain and try transact on it. If your counterparty is satisfied then cool, carry on separately in your own bubble.

As you continue to transact with the rest of the world you'll realize you were both on a fork as you eventually encounter a chain with a much greater block height and you figure out that's where the current consensus lies.

You will still have your your UTXOs on the main chain since the transaction won't exist there. You and your counterparty figure out which chain is subjectively valuable to you.

In reality you ascertain the provenance of the software you're using the same way as all other software. If you fall for a supply chain attack and use the wrong software then all bets are off, you're vulnerable to your attacker and anything can happen.

You don't know if there's going to be a reorg in as much as relying on quantum mechanics you don't know you're not going to fall through the floor. Nothing is certain, only somewhere on the scale of probable to improbable. Look at the available current hashpower and decide where the incentives lie.

Anyway this is getting into the epistemological weeds. How do you know the universe/simulation you think you're in wasn't created last Thursday? https://rationalwiki.org/wiki/Last_Thursdayism

[baby]

It seems like you don’t know about double spending attacks and how many times 51% attacks actually happened.