Start out easy on those sidebends. Low weights(25 pounds?), very short range of motion. Keep your neutral position, and try to really feel your oblique. You could also start out with the 1 Arm Farmers Walk. Your oblique will work, even if you can't feel it. Just be sure to start light, and to maintain position. Think of ab moves as gradually challenging your ability to stay neutral with gradually increasing ROMs and weights. Always remember the point of what you are doing.
In the past I didn't have a belt either. For the same reasons you mentioned. But I think over time not wearing one, I just developed another way to not brace correctly. Everybody else wears one for a reason, I guess. Try one on sometime, brace against it, and see if it feels good.
Start with a goblet squat if you're new to front squats. You might even work goblet squatting into your warmups or cool downs or whatever for more practice.
Sumo deads might be good in the future. They work the lower abs and adducutors pretty well. But if these areas are a weakness for you, your sumo technique is bound to be pretty ugly at first. And like you said, ugly lifts and back problems aren't so good together.
If this has been a 12 year situation, it will take some time and effort to correct. Usually more effort means less time. You've got to retrain your body. Be conscious of your neutral position all the time. Check out info from Kelly Starrett on Youtube. The videos are way more effective than just words.
The good news is that your back is going to feel better and better as you work on this.
Hi, forgot to reply but i have read your reply and appreciate it.
Will try a belt to see how it feels, cant hurt.
Already doing lots of Starretts stuff and other youtube found exercises and stretches, we'll see where it gets me.