First, the back band usually takes some tensioning to get it right for a given person. And the back band itself may need replacement to give proper support - Immersion Research and, if they can be found, Bomber Gear both makes back bands that people like. Or you can take a look at some of the cutomizable options in minicell available from places like Redfish. I would caution you to wait on that one though until you have time in the boat - once you've shaved off minicell it is hard to put it back.
The backband should give you solid support at the top of the pelvis/into the lower lumbar vertebra. If it is not doing that, you need to attend to the backband. If it is doing so, you need to attend to the rest of your positioning.
I am sure that you feel like you are sitting upright. But getting mostly upright and really being in a position for torso rotation are two different things. Once you get the hang of the later it'll help your back in a bunch of ways.
The Tempest has very kind stability. You will get used to it and learn to relax fairly quickly.
As to the knee pad thing - they aren't knee pads. They are supposed to be hitting you at the thighs, behind the knees, unless you want to do some damage to your joints. If you were actively pressing you knees up into the thigh braces it would be surprising if you didn't hurt. It increases the tension too much, and is a pretty nasty position for your legs and hips.
Most new paddlers lock themselves in too tight. All you need to do is to be able to reach the control surfaces (thigh braces, foot pegs and butt) when you need to manage the boat, not be locked in all the time.
One thing you will likely learn is to get the boat on edge by shifting your butt over to one side or the other rather than locking into the braces. It works as well, is if anything more stable and is a lot less taxing.
There should always be space for you to pull your legs out and let them relax - otherwise it will never be comfortable. But that means that you are just sitting in the boat without exerting any control, and that is probably not something you are comfortable doing right now.
You may need to do stuff like flatten out the seat pan a bit, change the angle and/or get some foam under your thighs to help extend the support of the seat. But I suggest that you get thru your classes before getting involved in that level of tweaking.
Skegs are not always your friend in current by the way, because they give the water more boat to push. Note that WW boats don't have any such thing. Your first recourse should always be a good edge, which you probably are not comfortable going for right now. It is something else you will be learning.
Kayak Deck Gear Bags
Touring Kayak Paddles
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