Is the outer envelope really necessary for the invitation?

We didn't- just a flat invitation with RSVP card.

We also did not use an outer envelope for our invitations.

We did not but we also had no inserts or anything. RSVP is done on website.

I think you mean the inner envelope. No, these are not necessary and very few people use them anymore. We didn't.

If your inner envelope is dark, you might need a lighter outer envelope.
If it's dark. Ask the post office because I might have zero idea what I'm talking about, and it could be a non issue that I've made up in my head.

No, you definitely don't need to add another envelope! It can be helpful if you have a lot of inserts, but it's not needed (and skipping one will help you save on postage too).

We only had the regular envelope Minted provided and one small envelope for the RSVP’s Inside. I didn’t realize that there had even been another outer envelope in the past. I don’t think I’ve ever seen an invitation with one. You should be totally fine without another one.

We only had one envelope. We only had the invitation and an rsvp card w/ envelope to put inside it though. It was light, and relatively flat.

We didn't have an outer envelope. I think they are necessary when the calligraphy is super hard to read so the post office won't be able to deliver. Otherwise, not required.