Okay, so this is high-level geekery, but: does the -an in the first word elide into the "et" (Forsan't?), making it a spondee, or are they pronounced separately, making it a dactyl?

Separately, dactyl-style. The full line is:

mittite: forsan et haec olim meminisse iuvabit

which scans as:

— ⏑ ⏑ | — ⏑ ⏑ | — || — | — ⏑ ⏑ | — ⏑ ⏑ | — —

and wow, I hate doing scansion by Unicode. I recommend overlaying it on the text by hand.