UPDATE: Yes, physical tickets can be returned even if they’ve been linked to MDE and have FP+ reserved, as long as they meet the other conditions of return (within 90 days, fixed date hasn’t passed). Source: spoke with Christine in customer service at Undercover Tourist.
I wrote to their customer service and asked but have not heard back from them yet, will post the official answer once it is received!
I know that you have 90 days from date of purchase to return a fixed date ticket that has been physically mailed to you as long as long as the fixed date hasn’t already arrived; but what I can’t tell is whether or not it can be returned if it is linked to MDE for FP+ purposes.
My gut feeling is that they can still be returned, but we’d like to know for sure before linking them just in case our trip (mid-May) ends up being somehow interrupted by COVID-19 (the only reason we can foresee needing to return the tickets/cancel our trip).