I would be nice to have a option where one could insert a followup message between two existing ones. lets say I have 10 followup messages and I want to insert a message between 7 and 8...

Quite a vital one this one I would say!

I think this would go against the expectation of subscribers and contribute to some possible confusion for some subscribers. I suggest using broadcast and/or create a new campaign to address this type of updating.

You can currently add/remove followup messages in any order. I think the request here is if you already have a day 7 and day 8 message, being able to insert a message in between to make it day 7, 8, and 9.

I'll probably not implement this feature. You can edit the order of the followup series anytime. If I make it re-arrange your series automatically then it would lead to more confusion. I might insert and move #8 to #9, whereas someone else might want to overwrite #8 and someone else might want the new one to be #9, and someone else might expect an error message.

When looking at these ideas we're trying to keep the control in your court in the least confusing way to the most people.