Yes, it will fit it.

Yes

No, did not fit 18oz narrow mouth hydro flask.

No. It fits the standard mouth, not the narrow.

Yes, works great on my narrow mouth flask and they are getting in longer straws than when I got mine. If you have any of the long Starbucks straws used with their cold drink cups those will fit also if you dip the end into really hot water before fitting into the top and you will need to cut about 1/2″ off the end.

Yes…it should fit just fine.

Yes. I have the 21 oz. Hydro Flask and it fits perfectly!

I don’t think it does. It fits the standard mouth on mine and the other on my daughters’ 12-oz standard mouth Hydroflask.