The cookie does send you to the webhp?hl=all version every time after your first visit but the fix is simple. Delete that extra bit from your url and press go to force a regular google visit. It will replace the cookie and make the new serps default.
Interesting... I didn't. Just tested a few minutes ago actually.
I clicked the link, then searched, then went to Google.com and searched again and I was back to the new SERPs. I was also sent to the new results after clicking on the Mother's Day image at the top? Maybe try that and see if it clears the cookie or something? IDK... I'm using firefox and currently signed in to my account, but I did delete all cookies this AM and did not search again or login again prior to trying to old version link.
Hmmm... must be something more complex (or stealthy) than just a cookie. No matter what I try, Google doesn't serve that browser the new interface, now. Not that I really want it for personal use -- I don't. But as an online marketer, I do want to understand the average user experience.