right. first up some pedantry: 暇 means “free” or “having nothing to do” not boredom. I agree that it’s expressing boredom in this case though, so I’ll use that when it makes the english more convenient.
on to the の case:
I think you’ve got the overall meaning, and as you’ve pointed out, it’s a sentence fragment that is a noun phrase. if you want to make it a complete sentence and you don’t have further context, the natural assumption is that there’s an implicit だ, but I don’t necessarily think you need to.
basically, other than trying to force がある into the picture, I broadly agree with your analysis.
and the は case:
this I disagree with (here we go again 
) because it has an entire unrelated clause infront of it (and 猫に手を貸したいくらい今 doesn’t function as a clause).
compare with 今は猫に手を貸したいくらい暇（だ）. in this case you’re emphasising the 今 by putting it first (and therefore making it the topic).
this I also disagree with. if we take the sentence 暇（だ）, it’s a “I am free” （私が暇だ）. the implied subject is “I” not “now”. 今 is an adverb here.
oither than those two points, I think I agree with what you’ve said.
nothing subtle about it. one’s a noun phrase describing the boredom of the moment, the other is a sentence describing how bored you are right now.
one of them describes the concept, the other one describes your current state.
if you don’t have context telling you what the verb is, だ is always the safe assumption since with anything else you’re effectively making half the sentence up.
“the (my?) current boredom” vs “I am bored right now” is the best I can think of at the moment. it’s a bit harder once you add the bit about the cat…