>>96388644
Ideally, for them this is just 3 dudes arguing with eachother on what's the best way to go at landing on the planet. Still this rules are general and for me to organize and think within a framework of 'what would make sense each of this guys would want to do'.

The way I was thinking to present it to the players was something along the lines of:
>Priest N1 tells the player that he's identified a good location where to land an archeopter full of skitarii. But he needs the place cleared.
>Priest N2 replies with "that's a waste of resources and it's not such a pressing matter. I have instead located the house of one of the guys whos in cahoots with the t'aus. How about we go and kill him first?"
>"Well operatives what do you want to do?"

Giving them the choice between doing agenda of 1 or 2. The choosing would be exclusive: time window closing, enemies reacting, the techpriest returning on their choices, etc.
Ideally, sticking to one approach, the campaign would funnel in that direction as "the way we've decided to do the thing"

>>96388684
1)Yes.
2)Given the theme/Philosophy of the techpriests, my idea was that the mgguffin would've been the explorator's removed memories of an "eureka moment" regarding the theological conundrum that "will" Versus "causality" creates within the order. Re-emerging later in life as a technovision, the guy would've figured out that chemestry is also causality based and people/fleshy bits are mostly made of chemestry. He would've quickly figured out that saying that outloud would've probably made everyone in the order mad. So he went in and hid his cogitator "forgetting" his newfound theory. I don't know if that's lame and they expect a tomb world tho.
3)Not extenively; If there are, I'd have them follow the expedition in a "Oh you're going there too? Craaaaazy! Given you're here spare some supplies right? We all friends here afterall ;)))" sort of way.
4)Average agri world with one-two population centers.