1. Yes, the technique to "Branch by feature" can be added on to any of the plans.
2. It sounds like you understand. But there are some tricky spots. It is not merely the presence of multiple dev branches that motivates the use of an integration branch. It is the fact that they must each have their code integrated and tested
before you have confidence they are stable. So if you have multiple dev branches but they are completely independent of one another, then there is not need to merge them to an integration branch first. By definition, the risk is low that one will break the
other.
I'm not sure an integration branch makes much sense between Main and Release branches. Technically, you can do it. But release branches are used for a different purpose than development branches.
David