Use a stiff fin and practice primarily kicking from the hips. I have some of the same issues. I tried splits, used them one day and gave them to my son. He loves them, but they did not work for me.
Think about the mechanics of this.... I use a huge long freedive fin ( Dive R) , which you must kick from the hip...mine are also the stiffest composite blades made, as I am a competitive cyclist, and my legs will push more water easily, than most divers would want to try for....the thing is, that as you push more from the hip, your lower back has a large increase in the need to stabilize your core area from this....in contrast, with stubby little Original design force fins, while you do have to kick from the knee, the "gearing" is very low, and there is very little torque on the knee for each kick cycle..yu just have to make a lot more kick cycles per minute.....and for the lower back, it is nice, because it is pushing you straight forward on the hips and waist, so the lower back does not need to stabilize anything to speak of....
Bare feet would be better still, but the speed potential is so low, that it is a foolish choice for a scuba diver. Ideally the OP should find someone who owns some original Force fins, and try kicking them for 10 minutes to half an hour, and see how they effect the knees and back for the next 2 days.....they could also certainly try so hip kicked fins like a very soft freedive fin--the cressi gara 3000 LD is as soft as they get...but my guess is that while they would be easy on the knees, they would strain the lower back to much with it having to stabilize the hip torque.
One of the thngs that could be very bad about split fins, is the poor "tracking" of the knee in the downstroke, due to the wimply double blades not really maintaining a direction that keeps the knee and muscles moving in the healthiest plane possible.... in cycling, a person with a kneee injury can get better by having a shoe/cleat position and seat position that keeps the knee in the healthiest rotation --I am not sure this will make sense to non-cyclists, but it is quite similar....there is an ideal knee motion you want to ensure...anything that has the lower leg swinging left or right out of the ideal tracking, will cause knee pain and inflamation, if not damage....The fin issue should be almost the same regarding the knee....Splits also work pathetically if the user trys to swim sideways to a powerful current--I think the twin blades lose so much of their directionality ( which was already poor), that they work much worse still in this scenario...and this relates again to the concept of the twin blades and poor tracking of the 2 wimpy blades--and what that could do to a kneee that needs to be "locked in" to an ideal movement.
I can come up a little sore... 
