We’ll start with your second question. It looks like you may have figured it out, but I’m not sure why you are using a relation from the rsvp sheet to the event sheet. You may have done this, but I would set it up this way: First create a form button on your event page. Open the form button and add a column component for event id, which will pull the value from the event details and populate an event column in your rsvp sheet.
For your first question, I’ve been trying to think of a solution, but I’m not coming up with a good one, I tried a combination of relations and lookups, but I get stuck since there is no way to link from the event sheet to the rsvp result that matches the current user. Personally what I would do is let the user RSVP as many times as they want. They at least have a chance to change their mind. Then in a sheet with each unique user, you can use a vlookup to get the last result for each user. From there you can compile the results however you would like.