Hello Edge!
I know I am taking this right back to basics when I ask you this, but I do have to be sure:
When you straighten and lock your legs, is your chest still sandwiched tightly and squashed against your knees and legs?
My initial reaction is to tell you NOT to lock your legs. Keep them bent and get more wrapping happening of the arms as far around the back as you can. Then, rather than TRYING to straighten the legs, think of your hips moving up to the ceiling as you release and relax the muscles in your hips and back and hamstrings.
This pose really is so much about the pull (and dare I say, less about the straight legs!). And the pull has to come from the biceps muscles. My worry is that if you can’t bend or wrap them as well as you could (say, with bent legs) then the pull starts to emanate from the shoulders which introduces risk to your back. The more you pull with your shoulders the more the arms straighten out, the more you round your back and hey presto we don’t have Hands to Feet pose anymore.
So even if you can’t get the elbows around the back of the legs, at least set it up so that when you pull you completely isolate the shoulders OUT of the equation. The further your arms are out to the side, the more the shoulders are involved and cause strain and stress in the neck, shoulders and back. So what you have to do is always pull with the biceps muscles and try to shift the shoulders up and back behind you, away from the ears.
Please get back to me and let me know any further distinctions. And together we can confirm your best plan of action…
Namaste
Gabrielle