Yes, those squats looked better (see how it's not a posterior chain restriction issue?), but it's unlikely that your thumb had anything to do with it. If it was a factor, it shouldn't be, and it means that there are restrictions in your shoulders/pecs that shouldn't be there.
The fact that you can keep it that tidy at 60kg heavily hints that this is a strength/control issue, like I said. You will be breaking position when the weight is too heavy to control. Paul had a similar issue.
Syla, did you hit a plateau on bench, is that why you've got decline as your main lift for chest?
I'm not sure that I'm following.Regards the thumb underpostion, it definitely is more restrictive for me with regards to how my arms/shoulders/upper back position is. So would it be ok to assume that this is an upstream issue feeding down, likely linked to grip change, which has highlighted chest/shoulder/t-spine mobility issues?

So, if someone performs a squat and I see the tuck under, and it corrects itself when I force them to engage their core, I can generally surmise that it’s probably a core stability issue. Not always, of course…..but it’s a start.

Good luck dude, hope it all goes to planMy aims are to now build up to my first powerlifting competition (early 2014), with the plan to be in the 74kg weight catergory.
