I work in a small town in northern Canada that has only 1 supermarket, which sells only 1 form of mussels: see beneath. There are no other options for mussels. It feels impossible to judge freshness by the little aperture at the top. I already know that I should choose the farthest 'Best Before' date as possible.
Unfortunately I don't think you have a good option. You can try to determine what you can through the small tight section at the top. You should also be prepared to take them back and ask for your money back if you get them home and the freshness isn't acceptable.
Once it is cooked it should turn almost orangy; assuming you haven't used spices that will alter its color; such as turmeric, cayenne, and etcetera.
if they are completely closed they should be fresh, after you buy them put them in cold water if they dont open it means they are bad. you will always have a few of them that wont open.
Don't have a solid answer for you other than some anecdotal experience. I've bought that specific brand from our local Costco numerous times and have generally never had an issue. They've worked out really well.The one time I did have an issue, by the time I went to cook them, I noticed the vacuum seal wasn't as tight as it usually is. As soon as I opened it, I noticed a stronger smell and a high number that did not move.
TLDR: I would look for a tight seal and as you said, find the furthest date you can.