Thanks for sharing the videos! Makes everything so much more clear. I agree with the others that this horse can benefit from a lot more of the “basics”, but I would argue less about hind leg engagement at this stage, and more that this horse needs to learn to engage her core and lift her back first. Mechanically, I don’t see any way this horse can get the hind legs under with that pelvic tilt and collapsed core in the way. Engaging the core will fix the incorrect tilt of the pelvis, which will allow the hind legs come under more.
Again, Simon Cocozza’s book will take this to the extreme stretch that this horse needs to work in until her back is stronger. Step 1 is to stretch the back, lift the abdomen, and engage the core, and step 2 is to work her this way in this stretch - leg yields and shoulder-ins with her nose down around the height of the upper part of the foreleg (stretching, not curling!). If done correctly, the lateral work in this position will give the horse no choice but to engage the core and strengthen this key area. From there, you have started to fix the pelvis, which can then allow the hind legs to start coming under more (thereby getting more engagement).
I don’t see any way this horse is strong enough to engage the hindquarters properly with her back as weak as it is. I would focus on the abdomen and back for now, then when that is stronger, you can get the hindquarters more under. Baby steps 
And it’s not linear - both should/will happen in a step-wise fashion, but if you can only focus on one, I would focus on the back at first. The horse’s hind legs are plenty active, and once the back is lifted, I think engagement should come fairly easily.
(caveat, I’m not a trainer or a bodyworker. Just someone that has owned and competed friesians and warmbloods in dressage, and someone who has recently dealt with a KS diagnosis in a horse with a tendency to move inverted but was still scoring really well at the FEI levels, so I’ve gotten really familiar with back and core health literature! Take my advice with a grain of salt!)