The writing and everything you mentioned in ME1 and ME2 were absolutely fine and made sense for the most part (the game has a little bit of space magic, but so what, it's fiction and isn't exaggerated out of proportion). The collectors had a point, i.e. build a Reaper to open the mass effect relay in the Citadel. Shepard's death and resurrection was fine, space magic tech FTW. No idea what Drew has to do with Tali's voice acting. The Reapers did not have a plan that made no sense, their plans and agenda were simply not revealed, other than the fact that they wanted to destroy/enslave all intelligent, spacefaring life in the galaxy, which made them very mysterious. Sounds to me like you just simply have a problem with this type of space opera fiction.
I have no idea why you are blaming Drew Karpyshyn for the ridiculous and terrible writing that Mac Walters was responsible for in ME3. The hole that you mentioned, the one ME3 fell into, was only dug in Mass Effect 3 by Mac Walters himself, it was not dug by Drew Karpyshyn in ME1 and ME2. You also seem to completely renounce the fact that nobody really had any problems with the writing in the Mass Effect series until ME3 was released.
- The Reapers were truly irrational and incomprehensible until Mass Effect 3,
- The force that drives the Reapers, which is to kill all organics so that they don't kill themselves (completely ridiculous and illogical, lol), was not mentioned until Mass Effect 3
- Star Child, "god" of the galaxy, was never mentioned until Mass Effect 3,
- that Shepard must pointlessy sacrifice himself, in order to prove that humans deserve to survive (what? lol), which ultimately ends the reaping cycles, was never mentioned until Mass Effect 3
and countless other ridiculous story elements, that were developed during the production of Mass Effect 3. There is absolutely no connection of these story elements to the story elements in ME1 and ME2, other than the fact, that the Reapers exist and have an agenda.
Not to mention that you bash Drew for the discarded ideas that the writing team had developed and are trying use this is proof that Drew is a terrible writer, although Drew himself even mentioned that "Some of the ideas were a little bit wacky and a little bit crazy". What? Seriously?
A few things:
Considering that the reapers were arriving anyway, it's a safer assumption that the purpose of the human reaper was simply to be, well, the human reaper. The Collectors were going to harvest enough people to build it, then the rest can simply be destroyed once the reapers finally arrive. Of course, this begs the question as to why the Collectors have to do this at all, since the reaper fleet is well capable of doing this themselves. The Citadel relay plan failed. It doesn't seem sensible to bother making this same desperate move again, risking the destruction of yet another reaper, particularly one fresh off the macabre showroom floor.
Shepard's death and subsequent resurrection is pointless and stupid, because it's perfectly interchangeable with a 2 year long coma within the narrative. Hell, even the characters themselves seem to be pretty inconsistent in the way they talk about it. Shepard says "I died" then it's "I almost died" and Zaeed talks about his/her surviving the Normandy's destruction. The time away being nursed back to health in some secret facility while the leaders of the galaxy sit with their thumbs up their butt was sufficient to force the temporary Cerberus alignment. Maybe it was all an elaborate ploy to make a geth unit unique by sticking on some broken N7 armor. Killing a character off, then bringing them back should provide something unique to the story, but it doesn't. It's simply a Shepard and Normandy reset button.
While ME1 does establish that the reapers are there to "impose order on the chaos of organic evolution", that seems rather incomplete. Why should they care? What's the point of imposing order? Are they simply galactic neat freaks that get off on making sapient life operate like clockwork? In all honesty, I wouldn't have cared one iota if they never answered these questions, because the ultimate goal behind their will to impose order was never really all that intriguing to me anyway. I would have been happy enough to just kill them without ever really knowing everything behind the curtain, because it satisfies the portion of my brain that's part lizard, part caveman.