Am I forced to build all branches when pushing to the target branch of multiples MRs ?

Hello, sorry to post as an "issue" but I found no other way to ask my 2 questions :/
The title of this issue regards the second question.

1- I have a multibranch Pipeline job in Jenkins previously set up only with the "git" plugin.
With this "Gitlab Branch Source plugin" Jenkins now detects MergeRequests as MR and build them specifically.
But I still have to keep the classic git plugin otherwise my job is not notified when new commits are pushed.
I this the intended behavior ? I was expecting to need only the "Gitlab Branch Source plugin", not both plugins at the same time.

2- From what I experienced with "Gitlab Branch Source plugin", a push in a branch set as target of multiple MRs will
trigger a build of all those MR at the same time. It is a problem for my job because my legacy application takes
a long time to build and I do not want this behavior: I would like the MR job to build only when there is a push in the
corresponding source branch of the MR. Or build it when I decide. Not upon any commit in the MR's target branch.
Is it possible ?

Regards (and again sorry to post here, I found no other support channel)