external mono sends and stereo returns

I posted this question in the Nuendo 5 forum, because I’m mainly a Nuendo user. But I have copies of Cubase 6+5 too and I think this is equally relevant for Cubase users. So here is my problem:
There is one thing that puzzles me with Nuendo (and Cubase) when using a mono send/stereo return combination for external effects. I would expect the send routing panner to crossfade between the levels of L + R going into the mono send. Instead I always get a dry signal on one channels of the stereo return as if it would come back fro the external FX device. This is not the case - the device is fully set to ‘wet’. I have to pan hard to one side, e.g. to left, to get rid of this. Panning hardly to the opposite side, e.g. right, give no FX send but just a dry thru signal. the left + right levels are not mixed to mono and then send out to the external channel.
I cannot figure out what I’m doing wrong or what would be the culprit here. My Nuendo 5 machine is a MacPro HexaCore with a RME HDSPe MADI card. Any hints what goes wrong or how to circumvent this are highly appreciated.

after one reply I wrote:
Thank you for the answer but I think that is not the last word. In the N5 manual on page 206 about mono FX:
“To use the pan control as a crossfader, determining the balance between the stereo sides when the stereo send signal is mixed to mono, route a send from a stereo channel to a mono FX channel track.”
But that seems not the case when using a mono send/stereo return FX (external, dunno about similar plug in configs).
Why? For example I have some older EMT reverbs with just mono inputs. When I want to use my EMT 246 I need to send a mixed mono signal from my stereo tracks to it and get a stereo return from the EMT. The mono send should have at default a mono mix of the stereo channel. And from there it is up to the operator to decide via panning what weighting between L and R he wants to have for the mono send. But this is obviously not the case with N5, or I have missed something crucial. This is what I want to find out: is there a solution (simple and ergonomic), is this an oversight by me, is this bad design or not a bug, but a feature …?

http://www.steinberg.net/forum/viewtopic.php?f=76&t=3887

Click on the insert and open up the control pain of the effect it by clicking the e in the circle. Then up the top of left of the panel that open you should see come vertical lines with dots at the ends, if not, right click at the top of the panel and choose ‘insert routing’ I think it’s called. Double left click on it and you will see one broken line and one joined line. Click on the joined line and it should break to look like the other line, and that will cut the dry signal that’s coming in one side.
I think that’s how it’s all named, I’m’ not at the studio and typing this form memory.
This is a handy thing, because you can put 2 mono compressors over a stereo track or group by using this function, you just put them 1 insert after the other and break the solid lines in each one’s insert routing by clicking on them.

Thank you for the detailed explanation but you described editing the send routing for plug ins (mono/stereo). I’m aware of this function but it seems non existent for external sends (mono/stereo) and this is my problem. There is a lot of hardware, mainly FX boxes and reverbs which have or only allow mono inputs with stereo outputs. As I mentioned, my older EMT reverbs are working this way - there is no stereo input. As long as there is nobody coming up with a solution I have the impression I discovered with that problem a big oversight. It’s fine with me accepting I do something wrong or stupidly but I have to be pointed into a direction to see it. So far I think it’s a design flaw or at least an ergonomic obstacle somewhere.