...45 and 46 are the last two, right?

Anyway, I've done a bunch of googling and I'm getting conflicting information about how to unlock these missions. I've seen stuff about maxing your relationship with Quiet, and having/not having certain elements in your Emblem? But those same sources say you have to do some of the EXTREME or SUBSISTENCE remakes of previous missions, when I've heard you don't ever have to do those to beat the game? I've also heard mention about a scientist in a cutscene you exile, and I'm not sure if I've seen that?

So what do I have to do? And if it involves maxing my relationship with Quiet, what's the fastest way to do that? Do you have to do Mission 45 to beat the game?

Just keep playing side-ops with Quiet if you don't want to play the harder missions

Not sure, I think my relationship with Quiet was already maxed, but seemed like doing the subsistence, or other missions like total stealth unlocked the missions for me. I just did the ones I could do, and the missions unlocked after that.

I don't know how I got 46 it just appeared as I did some of the missions before it. As for 45 you need Quiet at 100 buddy (rank?). I have finished both and still have side-ops to do and platforms to build.

Warning for 45:

The scientist scene is pretty obvious and has a build up. You'll know when it happens. I only did one of the repeat missions and managed to unlock Mission 46 so as far as I know you don't need to complete them.

I've also heard you need to listen to the important story cassettes and have a platform upgraded to level 4 but I don't know how accurate this is.

yeah doing a few side ops will unlock the story missions without having to do the extreme missions.