You have to. I don’t know if episode 4 can be saved. It’s bad bad. No underwater security? Darth Vader now speed walks? Snow speeders just hover oddly? vader doesn’t kill failures? Obiwan thinks a trench coat for a disguise will work? I literally cannot think of one salvageable moment? What if Obiwan let the water come in and they will escaped through the water?
Either way it would be convinient and/or bad writing. How so? Obi-Wan doesn´t have the opportunity to look for a better disguise (per your argument) because the writers deem it to be that way. So your rhetorical questions running nowhere.
Unless the Empire has Bantha costumes knocking around for R&R events then there’s no disguise he could have worn at all that would have hidden Leia as well. That’s not the writing, that’s just bad viewing of a scene that doesn’t require other interpretation. Also bare in mind he made it to within a few metres of the shuttles so it must have been an effective disguise.