No Cubes is now open source and needs help with updates and development. This mod has proven to be VERY difficult to update which is why it is now open source. Here are all the pages/information you will need. Good luck.
What if No Cubes had its own world type? The disadvantage is that it won't allow you to use the mod in servers that don't have the mod installed or existing worlds (Unless some kind of converter was made), but in exchange it could have working collision, a huge improvement in look, and no longer blocks that aren't really there. It would allow to save some additional data for every block. It could even be made to work with routable fluids.
An alternative mod idea instead of updating No Cubes.
What if all terrain worked like collapsible blocks like the ones in the Carpenter's Blocks Mod? This way all blocks would be solid and would look very similar to No Cubes. Would something like this be easier to code than updating No Cubes?
I seem to recall edge blocks only being made into simple slopes earlier on. Might be good to take a step back to that appearance temporarily, so we can more easily tell if we get something like the routable fluids in the mod you linked input in (and if we really want to go insane, have it flow according to finite liquids) and make it easier to solve the issue with invisible holes. If it is built right (ie. having it dynamically rather than just using a preset block), it would make changes to get the current 1.0 appearance easier. Might even be possible to have the mesh and render run off the same algorithm to have the ground be where you actually see it.
I'm a dev & I'm now working on a port of NoCubes for 1.12.2. I've already got it working and fully compatible with Optifine & BetterFoliage. Join the discord server if you want to help with the mod/ask questions/view the progress on it.