Guys, ...
What do you make of this?
Its complete bullshit.
1. Is it possible to move any of our coins held in SegWit addresses to legacy addresses (so that they would belong to the legacy chain if the above attack does happen)?
What do you mean "Legacy chain?" There's only 1 chain. SegWit addresses are just addresses displayed in a different format. They both are derived from the same type of private key.
Does the fact that, at some point in their history, those coins were held in SegWit addresses, taint them in any way? Would they still be considered as fully belonging to the legacy chain once moved to legacy addresses?
No. There is only 1 chain: the Bitcoin Blockchain.
2. Would you do it? Do you think it's worth it?
Your SegWit coins are not in any danger.
3. Do these questions even make any sense?
There are no bad questions, so yes.
4. Am I wasting my time with all this?
Yes. In short, a 51% attack would have to be carried out in order for the "anyone can spend" ruse to come to fruition. So they are just as safe as any other bitcoins in any other type of address.
Any time Shelby writes anything he's basically just saying "PAY ATTENTION TO ME!!!" Each and every time.