Yes, posts and comments are upvoteable even when not subscribed to their community. Subscribing only affects what content you see when you filter your feed to "subscribed".
Yes, BUT I think they only count if your home instance is properly federated with them. If that connection isn't there, it's my understanding that your upvotes are only visible to those in your own instance.
I think you can, actually. One instance can have an entirely separate comment section. It's almost like the users of one instance are shadow banned from the other.
Actually, I think I'm a little wrong: that happens when one instance BLOCKS another, and then you're commenting on the blocked instance from a THIRD instance? It's all kinda complicated and I'm not sure I grasp it fully.
This is sort of incorrect depending on what you mean by "properly federated". Lemmy instances don't actually filter AP messages except for from blocked instances. So if you found some way to send an ActivityPub "Like" without actually viewing or subscribing, it would indeed still count. Keep in mind, Lemmy isn't the only software that can create and publish "Like" activities.
Actually I don't think downvotes federate with kbin (I'm not sure about between Lemmy instances?)
Downvotes are not intrinsically part of the ActivityPub standard.
...but yes upvotes federate just fine!
In the current form of the ActivityPub Protocol, downvotes aren't accounted for and thus don't get communicated to other instances, regardless of whether they're kbin or lemmy instances (or any other fediverse frontend).
Additionally, some instances have actively disabled downvotes.