Help with Supine Flare Leg Lift and Pancake Lower Back

I've been working the Middle Split Series pretty much every week for four and a half months now and I feel like I'm seeing progress on some stuff, but the pancake is killing me. I have zero flexibility in my lower back (I guess from sitting a lot?) and need one and a half blocks just to get it straight when sitting totally upright in a straddle. I can bend forward about 6 inches before totally losing my APT.

Also despite having no problems touching palms to the floor in a pike with straight legs, I can't seem to lift my legs more than 60 degrees for supine flares (or when standing).

Is there a daily limber stretch or any other advice or exercise someone could suggest for me on these? I feel like I'm seeing zero progress on this. 

(for added background I've been doing TB for a month and FS for about two and a half months)

Hi Drew, sitting means tight hamstring and calves. Do you practice also the front split course?

Yes, I've been doing the front split course for the past two and a half months 

The major problem here is the hamstring and the calves in your case, so probably a daily light stretch for them it is the solution. they took an impressive amount of time, be patient.