Sorry to belabour this, but I want to get it right the first time.

I saw the following quote on a website, which seems to conflict with your
real world experience.

[quote]
The skimmer suction line MUST be hooked up directly to the pump. The skimmer
will only work with suction under pressure. It will not work by gravity feed
into the vortex, (water takes the path of least resistance). Almost no water
will flow from the skimmer to the vortex if you install the skimmer
incorrectly. All you need to do is build a manifold on the suction side of
the pump with two valves. Valve #1 is to control the volume of water from
the filter, while valve #2 is to control the volume of water from the
skimmer.
[END QUOTE]

I thought I would test this, so I plugged the overflow (skimmer) in the
bathroom sink and filled the sink to the brim. When I unplugged the overflow
I got some decent skimming of my test debris. When I partially opened the
bottom drain I continued to get skimming, but perhaps a little slower. I
could not open the bottom drain fully or my skimming experiment would have
been very short lived. Not very scientific, but the skimmer still worked,
even with the bottom drain in operation.

I had planned to plumb my skimmer/bottom drain much the way the bathroom
sink works. But after my experiment, I think I will run both pipes into a
Wye with a gate valve on the flow from the bottom drain. So I can control
the flow of the BD and address my frost concerns.