I think the solution is
Then I think it will work
You see the problem is that i would like to show all "updates" related to the selected item in grid1.
After re-reading your opening-post "I would like the record to be autoselected " --> as far as i know this is not possible: you need to select the record manually
"and the second grid then to be populated" --> this is no problem after the click is done manually (see my previous post)
"or changing from double click to single click but then Mendix tells me that Allow Selection cannot be true. please assist." --> When using singleclick, selection is not possible because this would trigger the newEdit form to be opened.
What I think you're looking for is a kind of "navigation", aren't you?
In that case, I think you need 2 microflow triggers, previous and next and a microflow that (initially and after "previous and next") will fetch the data.
Something like this I made earlier: had a datepicker and with microflow buttons you could choose "previous" and "next" where only the records of that date were retrieved.
Hopefully it helps?