What happens with Storm's End and Defenders Mel out at the same time? Mel says claim an Additional power, but Storm's end stops you from actually claiming power for dom, right?
Defenders Mel and Storms End
I don't think Storm's End stops you from claiming power for your house if you win dom. I'd say it replaces claiming 1 power for your house with two characters claiming 1 power each. Any additional power claimed for dom is unaffected by Storm's End.
As Ratatoskr said, these cards don't interact with each other. Storm's end is optional replacement effect. Mel's effect triggers as a passive effect later in step 4 (gives you 1 power whether you used Storm's end or not).