It's WAY too vague and has very little to do with each other. Yes, both have to do with selfishness but selfishness has different varieties. Mordu was greedy and it led to his destruction while Merida just wanted her independence and rights as an individual by breaking tradition that would unfortunately would lead to a goofy war because the men in the movie are idiots. It's not relevant enough to relate to Merida's situation because it's completely different and isn't very convincing.

Extremely!

The spell happened twice over the same thing, family strife, what more do u need?

yes, that was why they made Elinor tell it. to give us the message that selfishness can cause utter chaos and ruin an entire kingdom.

They kind of beat you over the head with how the stories are connected. Merida's tapestry is torn just like Mor'Du's stone slab, they're even broken down the middle separating the families. IT'S SYMBOLIC? GET IT?!