You are shooting a big slow bullet in a big case (straight)
Primers can affect a trajectory as they can be several thousand PSI between them on firing from a modern bottleneck case.
I doubt anyone has studied 45-70 and in your case you are loading very low (not a problem but for a bottleneck at least it can get into a too low issue and some odd affects)
So, as I have told them at work more than once, we spend a lot of time (and in our case money) and still not get a straight answer or we can just accept it, do what we know will fix it and go on with life.
I have had two occasions where a 10 grand plus engineering job was done and they still did not give us an answer. There was a way to go around the one, the other has had all the "fixes" done to it and it has to come apart to see if it stopped it or its still a mystery.
Far more often I have just done what it took to fix it or we got equipment in that was a sure fired solution.
You can decide what level you want to put into it.
As you probably have seen, there are facts and then there are opinions and often no two shooters have the same opinion.
If research has been done in all areas its proprietary and never been revealed. Mostly I think they put money into it if its a production problem and don't care if its not.