This mod is a godsend, I can never remember the inserter lane rules, thank you so much!

It would be lovely if we could have the indicators showing while building and for ghost inserters too.

Thanks. No one can remember the lane rules :)

I've added support for ghosts. I somehow remembered that this wasn't even possible and therefore I didn't implement it, but it was quite easy now after revisiting. Stupid past me.

Regarding held: too much functionality required and one of the reasons I've created this mod to get rid of from the original mod, as this got very ugly in big bases (https://mods.factorio.com/mod/inserter-visualizer/discussion/6580757f7185656e9f5427fb).
You can write your idea how the held functionality would work in a simple/efficient way for the player. In general I wanted to avoid any shortcuts/toolbar buttons if possible. Need to think a bit more about it, but probably won't happen soon - if at all.

Now that's what I call fast response time! Thanks!

Regarding ideas for held items, I have no idea of the technical limitations with this, but I immediately see the challenge with how silly it can get with the abundance of mods that let you have all sorts of inserter configurations... But one idea, squarely in the category of "so crazy it just might work", is to create a hidden utility surface and copy over the belts from the target tiles (or worst case like the whole 5x5 or 7x7 area you're currently pointing at), and then actually place an inserter there to work out the calculations.

No, I understand now, there's no good way to find where the cursor is if you're not hovering over something that already exists... I can't even find any way to read the current orientation of a held item, so IDK. I could sort of picture a hotkey that opens a slim window with a simulation showing all possible belt arrangements, or maybe if you hover over a belt you can see both the pickup and drop spots just for that, but it's not feasible without knowing the held item's rotation.

Wrong again! https://lua-api.factorio.com/stable/events.html#CustomInputEvent got a cursor_direction property in 2.0.35, found via this forum post: https://forums.factorio.com/viewtopic.php?p=667067

So it is possible, but requires a hotkey.