The 20 people part may just have been our server, but it has only really bumped up to around 30-40 people.

The Join In Progress issue can happen on any mission. I've never really found a rhyme or reason to when this issue occurs but when it does loading in can take anywhere from 1 to 30 minutes to get past receiving data.

This issue will happen on any mission. Particularly prevalent on non server switched clients with sometimes Players being refused entry until the host passes briefing been told that "Waiting for Host". If not quite often everyone will see themselves as in Briefing but nobody else and quite often it will take a majority of people to be in the briefing screen before everyone synchronizes.

Oh, no mods are needed to replicate the issues.