1) im not sure
2) doesn't particularly matter, I'm using a 265k and it doesnt support HT, it works just fine. That said if you are gonna be using an ancient cpu from before the multithreaded days it might be problematic.
3) your cpu being an F sku does not support intel quicksync for hardware transcoding, you will need to use your 1660 with nvenc for transcoding
man every used cpu i recommend ends up being sold out by the time someone else needs a recommendation 😅
if you don't already have your 9400f in hand already i would look into a 8500t on ebay. It has an igpu which is great for troubleshooting if you ever have a gpu issue, support for quick sync but most importantly you won't need the dgpu.
I could be wrong but it looks like your usecases don't really need a gpu as strong as your 1660 and on top of that you would be saving a lot of money on electricity without a dgpu sipping power during idle 24/7