If the opponent is attempting add one of their attacks to their momentum during the end phase and I use Miserable Existence to cancel that gain, will I be able to take a card out of my card pool to my momentum?

Yup. They'll still end up adding that attack to their momentum though as part of the card pool clearing process.

So if I don't have any cards in my card pool I don't get any momentum from that source, correct?

Yes, that is correct. Miserable Existence needs to mirror the source, so if your card pool is empty it won't really do much in this case.

Can miserable existance be used when the enhance on kunai is played, to stop kunai being added to my opponents momentum as the cost of its ability. If yes, does the ability still count as being played, for the purpose of adding a card from hand or card pool (such as the same kunai) into their momentum?

babelfish666 said:

Yes, Miserable Existence may be used in response to Kunai's cost. If you do that, then the ability was still attempted (may not be re-tried in the same enhance step), but the cost was not paid so the effect will not resolve.

Note though that for this to work you have to have a card in your card pool to add to your momentum in place of the Kunai, otherwise no replacement happens and Kunai will still be added.