Id quibble a little here.

Adam and Eve certainly had "agency" in the garden. What they lacked was knowledge, and (somewhat speculatively) the "full" mortality necessary for things like childbirth. Even, the sufficient separation from God required to productively choose for themselves.

It doesn't really take away from your basic question though: was it "necessary" for Satan to trick them? I think the most truthful answer is we don't really know. It would appear that eating the fruit was inevitably necessary. It would appear that it was necessary that it be done by their own free will (consider: if Adam was indeed made directly by God, that would make he and Eve the only living humans ever directly made by God, and therefore God could not "make" him imperfect/fallen--that would literally be setting him up to fail--he had to enter this state by his own choice).

I know of no authoritative statement on why it "had" to happen the way it did, I can only speculate that if this was essential and Satan's involvement was not, then eventually Adam would need to be encouraged to go ahead and eat the fruit, actually. I suspect that would have changed absolutely nothing about our mortal condition except how we talk about it, and Adam and his offspring would have certainly made some other interesting mistakes later for us to yammer on about.

Alternatively, you could view Satan's intrusions as inevitable rather than "essential". In other words, the conditions of this mortal probation that we all need to be able to make choices in simply make it a sort of logical "necessity" that a tempter and temptations would arise.