It depends on how the vote on corresponding ideas goes. Right now they are not at the top of the list.
But if CN is a commercial success in the future, I can very well imagine making CN for other platforms.

I don't see a reason why the attachments should be stored in the DB. So most probably they will be stored in a special folder (changeable of course). And you also will be able to attach shortcuts to files, as alternative to making a copy.

But what is really difference between "color" and "category"? It is just name and that's all.
Calling it "category" would be more fair since this is what it is.

I understand that it's tempting to do everything using tags as universal mechanism. But I think there is a point where one should think about introducing a new entity just because it will be more convenient for the user. Just imagine 100's of tags with 100's of subtags, some of them colored... this doesn't look very usable to me.

I think everything will be much simpler as soon as Recycle Bin is implemented, the unsaved notes would just go there.

It is possible, but introduces possibility of db corruption, when there is a tag but no boolean flag and vice versa. Treating booleans as periodically updated cache could help though.

It used to be very slow, but now it's ok at least for me.
If anyone else experiences problems with UE, please upvote and leave comment here! Thanks

Are you willing to give up fast search for that?