Congratulations with your weight loss and with your strong will to keep up an intensive exercising regime! It is not easy.
Dermaneedling can be used on raised scars but not on keloids. Actually, I have heard about cases of keloids being improved by dermarolling. However, keloids react unpredictably and dermaneedling, excision, laser treatment or any other serious disruption can trigger the keloid to grow even more. Steroids (e.g. Kenalog) cause skin atrophy and are therefore used to reduce keloids.
Try a 1.5 mm dermastamp with 35 needles on a very small part of your keloid and you will see what happens (it may help). There is no other way to find out than trying it.
If your keloid reacts OK to dermastamping, apply the pressure massage described here:
https://http://forums.owndoc.com/dermarolling-microneedling/what-to-do-about-hypertrophic-scars/Use the 1.5 mm dermastamp on your other scar as well. It is not a keloid so there is no problem. Stamp it densely every three weeks. Our 0.5 % Tretinoin cream can also help with diminishing the redness of your scar but it is not completely necessary. You can buy just the dermastamp.
Concerning your other question:
Dermarolling can improve mild skin laxity, provided there is not too much excessive skin. Dermarolling cannot get rid of excessive skin after major weight loss. Skin folds can only be fixed by plastic surgery. We do have customers who successfully tighten their post pregnancy mild laxity of abdominal skin.
From our experience, the best results for sagging skin is the combination of a short-needled dermaroller (to speed up the turnover of the skin) and a long-needled dermaroller (to trigger the formation of new collagen and elastin). We recommend using a regular 0.5 mm dermaroller (roll up to three times a week) and a regular 1.5 mm dermaroller (roll very densely every three weeks).
Significant weight fluctuations are really bad for skin laxity. You are young and if you stabilize your weight, your skin will very likely tighten by itself with time. It can take a year or more. Your weight loss was significant but not so much that it needs surgery.