The problem is, that probably the "premise(s)" was (were) NOT premise(s) while building up the story. I am rather sure that the authors didnt sat around in their office meeting rooms for two weeks and thought about a NextME and speculated about possibilities to leave the milky way and when they had some real good logic chains they said that would be a good story.
In my opinion they sat around and felt f***** up by the ME3-ending and how they could avoid killing the stupid ME3-ending-**** cause they were not allowed to and so they decided to move to andromeda. And AFTER they decided to move to andromeda they started to find any kind of weird reason why this could happen. Even worse is the possibility that the authors WERE TOLD by managers not to get in conflict with ME3-endings and that they have to leave the milky way.
In other words: I think in terms of builing up the story the premise was to move to andromeda. The reasons why they did it in the story is probably the result of that premise.
If this is true its only a question on how much time they had to come over with that explanation stuff. I even think it is possible that they dont give any precise explenation but only very vague hints why that has happened. "Now" they are over there after a "looooooooong journey" and have to deal with it.
All thoughts you made have been discussed in several forums I visit for talking about ME. Of course the reaper shouldnt only be capable to follow to andromeda but even have alreade been there. ME-wiki tells that the max. travel speed of the reaper is supposed to be around 10.000 times of lightspeed, which means they would need about 250 years to andromeda. For an intelligence that is able to exist several MILLION years thats something like a few hours if we compare it with our life span. This is something any character could reckon within the ME-universe who tries to collect information about the reaper. So... running away from reaper would be a stupid reason.
So I dont expect any "intelligent" solution from the ME-Andromeda authors for this question. But if they surprise me and do come around with a real nice story that makes sense... I would appreciate it of course. But because I like it much more to be surprised than to be disappointed... I dont expect anything
- because therefor too many SciFi-stories in the last decades have been smashed by the authors themselves in trying to let everything be sooooooo awesome and soooooo huge, etc etc. The often just forgot this stupid little thing called logic. But... who cares. Consumers are dumb. Aren't we? 