You've been doing OK haven't you? As for recently obtaining lower back pain it could be anything, and could have been building up over a long period of time and is commonly caused by lack of core abdominal strength rather than any problem with the back muscles. Get a quack/physio to set it on the straight and narrow first.
I don't know enough to help with managing anyone's back pain but my own. My back pain is different to you everyday garden variety lower back pain, but the management plan is probably the same. Here's how I have handled it over a 24 month period.
1. Spin hands on hoods. Is it comfy? No? Adjust position according to what body sez is right
2. Climb hands on hoods. Is it comfy? No? Adjust position according to what body sez is right
3. Spin hands on hoods. Body say to adjust anything? Trim position, go back to step one. Repeat.
4. Ride lotsa miles with no back pain. When strong enough change handlebar height by changing stem angle and resetting seat position to suit. Oh yeah, you're back at step one. Repeat until stem angle at zero.
5. If you aren't already on the drops, start getting down more freq until comfy. If drops riding causes backpain then raise stem angle until comfy and then progress down.
6. Don't slouch when drinking. Especially important when you reach your umpteenth beer. Stand with one hand behind back to encourage best posture
On your freebie thingie, the obvious one to go for is the bike fit. Personally I don't trust anyone except myself to do that, and I get the feeling you're the same. To go for number 4 you would need to have at least some idea of your training zones, which implies that you need number 1. Do you trust these guys enough to do the testing properly and provide appropriate training zones? If so, do that. Any decent coach should eb able to build a program based on the results.
PS. I know nothing.