I know the passphrase at slot 0, but I have no idea, what’s at slot 1. Maybe it’s some old passphrase of mine, maybe it’s something that was created automatically. There’s a post at this forum (cannot link to it, weird), where someone hints that installer sometimes does that, when encrypting multiple partitions. I have just one encrypted partition but also encrypted swap.
Is it OK to remove the content of slot 1? Will I be able to boot up my system?
Add another passphrase (to yet another key slot)
that way you have some redundancy, should you decide to remove one slot and pick the wrong one.
If I remember correctly (but this was someone with problems with an Ubuntu system) one slot was for a key to open another partition and one was for the passphrase.
It may well be that one slot is for your swap.
There are ways to analyze/dump the keys and see what they are - but right now I know as much as you do and would need to read and search to find out … just as you. 
I do not think so. (Please search the forum, and the arch-wiki)