Ah, sorry about that.
I see you've already found the Using the Rest API with Custom Tags page. As you pointed out there, you would need to know the key for each custom tag.
You can use the following filter to get posts where a custom tag is set:
posts/for/metadata/key/[key]/value/[value]
Note: in the case where you have an attribute tag, set [value] to true
You can use the following calls to set and remove tags:
metadata/key/[key]/reset
metadata/key/[key]/set
Unfortunately, this only covers one and a half or your use cases. Can you please add your request to the Customer Ideas board?
Thank you,
Yuri