I did squats and deadlifts with my shoulder blades spread (as a trainer told me - every exercise, he says, the shoulder blades should be pushed back spread). That correlates with some weird twitching between my vertebra and aleft shoulder blade. Doctor found after X-rays that it's probably due to a mild damage of my intervertebral discs. Can that be a causation: that is, spread shoulder blades during deadlift lead to this? (I did deadlift and squat only about 5 times this year)
You wrote about x-rays, so I assume that you are in contact with doctor. So I'll skip that part.
For squats, and deadlifts - general rule is that you need to move shoulders back, with shoulder blades moved down, and pitched together. Reasons for that comes from shoulders, and back. That is good for shoulders, since they are open, and can take wider motion range, without later problems. Stretched back gives more stability, and makes muscles to take weight.
My shoulders mobility is/was poor, so I've developed squat with wide grip. Problem is that I can't put the bar lower, on the other hand - I do 3 sets with 4-6 reps with 140kg as a normal training for maintaining squats.
Personally, I don't think that squats / deadlifts are causing the issue, it can be with you for long time. That exercises just made it to show up. It could happen if you do that without trainer, then you can bend spine, and that could cause the problem. Note that, you can "develop" that problem with many other exercises - like over head press... People tend to bend spine too much, to support the move. Even pull ups can cause that problem - if done in cross fit / dynamic way without earlier preparation.
Kelly Starrett is concentrated on spine, look for criticism about him to have wide view. But the way he describes how we should take care about spine - must know. Order of stretching muscles before exercise, to stabilize spine, properly support it - as wrote - must know.
Listen to your body, listen to your doctor, and it will be OK. 10 kg is a pack of water. So if you take two of them from ground it the shop and put it into the cart? We do a lot exercises, gym is to teach us how to do them without problems. That was what your trainer did telling you about shoulders.
Fingers crossed. Hope it helps.
Can't say anything about the twitching issue you're having, or what's causing it - that's something for a physiotherapist.
As far as pinching/spreading your shoulder blades goes - you should be retracting your shoulder blades for both the squat and deadlift - as in imagine trying to hold a pencil in between them.
This creates tightness in the upper back and prevents your back from rounding during the lift, and allows you to drive from your hips. Spreading your shoulder blades leads to a loose and rounded back, which is not what you want to do. The idea of creating tightness in your upper back, along with bracing your core is about protecting your spine and creating stability and efficiency for a safe lift.
If it's hurting, something isn't right. Stop and get it checked out.