Oops! Sorry, forgot to get to this second part...
Also, with your example about the 2 of cups, you talked about using the filter of emotions to make decisions (which can lead to wrong decisions), so since the 10 of cups also contains cups, is it the best love cards since it's the end of the suit?
Emotions aren't always reliable, but (1) I think we can agree that you do want your heart to be part of a relationship and (2) while there are high and low tides to water, it can run deep. If it runs deep enough, then it won't dry up; and its ability to flow becomes less about being moody, then being able to adapt to new containers. That image of the family under the rainbow, suggests waters deep enough to last; to unite the two lovers, to unite the family. To, perhaps, unite the community and the world. In the Thoth deck, the card is called "Satiety" meaning that you couldn't be happier or more fulfilled. And it's positive meaning is that love (those cup emotions) unite body and spirit.
Remember also that while 10 is an ending, it also adds up to "1" suggesting that endings lead to new beginnings. The kindness, forgiveness and generosity of the 10/Cups, leads to that overflowing Ace, which can quench anyone's thirst.
As for 2/Cups, excepting the 2/Wands, 2's are that moment
before you make a decision. It's a gathering of information, or in the case of 2/Cups, gathering feelings. Exploring the connection, if you will. After this, the decision will be made: invest more emotions and strengthen the connection, or not? And if the 2/Cups is part of a relationship reading, then both sides have to make that decision. Otherwise, you end up with the puppy situation. You committed yourself to the puppy, but the puppy didn't commit to you. That's the problem with relationships. They require the other person to be on board, too.
2/Cups says that the other person feels that connection as powerfully as you do, but it doesn't promise that they will reach the same conclusion. Which is a bigger issue, I think, then whether or not you should make such a decision based on emotions. It's really important to keep in mind that, unlike other 2 cards, yours is not the only decision that matters here. However powerful and profound this connection, nothing can come of it if one of you invests, but the other does not.
Does that make sense?