Quoted:
I know it's not a 100% answerable question as it's officer discretion and sometimes cops don't like civvies with firearms, but if an officer were to stop me, and ask, would I get in trouble for having it stored in my backpack, in the back seat?
No, it's not the officer's discretion. Your age is irrelevant. Case law strongly supports the lawful carry of a firearm in a backpack on the back seat. Officer's discretion would be to pull you and/or the backpack out of the car if you are stopped, not whether to arrest you or not for a lawful act.
Keep in mind that while FHP and most large agencies should be current, my experience is that smaller agencies are not always up to date on current laws for a variety of reasons. Many cops still believe you cannot posses a pistol if you are under 21 YOA, This probably comes from stories of having parents buy their pistol for them when they were in the academy at 19 YOA. Same thing with the "three-steps" bullshit. Print out FS 790.25 as well as the definitions for that chapter and keep them with you. If it becomes an issue, just advise them that you stopped by your local PD and they explained to you it was lawful. This is why it pays to seek out a firearms friendly local cop and make friends with him. You may not beat the ride, but I can assure you that you will beat the rap.
PS: Don't leave the backpack where it is visible and will get stolen after they smash your window.