Hi!
Thanks for working on it this quickly! I've tested the feature in Collection and Cube Mode and it seems to work mostly pretty well, great work!
There's a bunch of cards that don't show all or any possible Alt Art Version, often when Best Selections or similar prints are involved. Ain ST05-002 for example doesn't have selectable Versions. The price breakdown does show the versions. Other cards have duplicate versions, often when PRB is involved. I assume this might be an issue with TCGplayer / cardmarket having different names for the same Version?
The "adjust price manually" option does work great to circumvent this in the collection though. My only suggestion would be to always make this available. It only appears once you've selected a non-base art version, so you can't 'correct' the price for card like, again, Ain ST05-002, which is missing it's other versions.
I do really like the cube system on here! Even just having a deckbuilder that supports cube at all is great, being able to even to test draft against people and / or bots is the cherry on top! I think part of the reason cube isn't as popular for many newer TCGs is that especially japanese TCGs aren't really designed with Limited Formats in mind and most people don't want to add extra rules to make it work. It's also not something that works well online, which makes it a bit less accessible and also means there's not a lot of content about it out there to even get people started. So having a page like this were people can get inspirations is a great step to maybe convince even a few people to give it a try =)