Pretty interesting... though I happened to click the wrong button and dundundun, I engaged the systems defense fleet, turned renegade and the ]I[ purged my chapter...
Also It looks like you cannot take every chapter they offer... strange...
Yeah, you gotta pay attention. The game is still work-in-progress.
A few tips and tricks:
At the very start, load 1-2 tech marines, librarians and apothacaries to each of your strike cruisers.
don't forget to transfer vehicles, as all your vehicles start on your homeworld (unless you're a crusading chapter)
Then, pick 2-3 escorts and place 1 techmarine in each (optionally place 20 astartes too). Use these three escorts to scout and pick up STC fragments and scout ruins/artifacts.
to do either, you need to unload marines (for STC, only a techmarine can pick them up)them onto the planet.
ATM, the interface for that is ONLY in the chapter managment screen. NOTE: marines are listed under the company or "office" they serve, not the ship they're on. So unless you transfered them specifically to another company, all non-assigned apothacaries will be in the apothecarium, psykers in the librarium and tech-marines in the armorium "group".
Raiding is preferable to attacking.
You can't use armor while raiding, but since it's a surgical drop-pod strike, you'll face less enemies.
Attack only if you're really strong.
Also, the ]I[ will be a royal pain the butt.