I cannot load my party presets from the party archive as soon as play in co-op. No presets are listed and all button are greyed out. As soon as I'm solo again it works like it should.
When you join as a team member, you should load your build before following to the adventure. On subsequent adventures, you should be able to load at the first screen before starting the adventure. This fails fairly often for me, particularly in the EttSC modules. I think this is as good as @Farbs was able to get co-op to work with the existing program.