Should not be any problem using the water. It will contain a lesser amount of free oxygen than running water and may be slightly more acidic, but that only affects aquatic life.
The decaying vegetable matter(leaves) will add nutrients and get your plants off to a good start.
I would prefer it antime, versus gray water.

Great, cause it's definitely the most handy thing. I'll just pretend it's compost tea- haha.

You can get a little aerator for the pond. Some of them are pretty nice- fountains with lights and the whole shebang. I don't see why it would be harmful for irrigation though unless it was polluted.

I think I might put some fish in there after I clean out some of the muck into the compost heap. My sister swears by a few koi or goldfish for mosquito control.

The only manmade thing that has ever been in the water are mosquito dunks, but I think those are safe for plants.

I always thought it would be really entertaining to stock the retention pond behind the house with Asian Carp. Those are the ones you may have seen on the news that jump out of the water and fly into passing boats. They'd probably grow legs and walk out of the pond and start eating people.

Here is a link that might be useful: Asian Carp