The only one that really threw came in the beginning of the movie . when Cassian gets contradictory orders to kill Galen rather than rescue. Why go at all then ? The whole reason is to find out if the new weapon is real and how to defeat it . if you just kill him you let the empire know you are on to them while learning nothing at all !
IMO, at the time they knew he was WORKING TOWARD designing a super weapon. I think that were just thinking if we have this chance to kill him that would stall the process.
Meanwhile they are attacking the shields themselves instead of the ring shield generator thing which of course is what they end up crashing the two death stars into to finally take down the shields.
It seemed like they were doing both. The point where it opens must usually be a weak spot but that is conjecture.