Is this a true union ball valve?

Yes, it is a true union ball valve though I'm not familiar with the brand so I don't know how high quality it is.

Yes it can be used for your siphon drain but the drain will be easier to tune if you use a gate valve.
 
^^^Exactly what he said^^^

It will work ok, but they tend to be very stiff and difficult to make fine adjustments with. You will be much happier spending a couple more $$ and going with a gate valve.
 
I use the same ball valves, they are better than the ones Home Depot sells. I also agree with a gate valve is better for your application
 
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