Holy crap that bypass is sketchy. And mixing the coolants will cause corrosion and can cause sludging.

I would revert the bypass back to the stock valve, it's <$100 from a dealer. Less online. What you have is just not reliable.

 

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Can you explain why it's not reliable? I didn't install it, so I'm not trying to defend it. I'm just curious what's wrong with it. Is it the screw-drive worm clamp? I heard that those can loosen up eventually, unlike, say, a spring clamp.

The guy who did the bypass said that the old heater control valve was cracked, so he tried to source another one from the junkyard, and that one similarly cracked because it's all plastic. Is there perhaps an all-metal, aftermarket part that I could buy?

 

On the green vs. red coolant, I have heard on the intertubes that mixing significantly different types of coolant can cause problems. So I avoid doing that.

But I've been running pure green stuff in my '91 for over a decade now, with no problems.

 

The heater valve bypass may be just fine. It should be easy to see if it is leaking, if it is a problem.
I re-uploaded your pic for clarity. Weird how this site sometimes lets you post pics and sometimes not.
The thinner pipe-to-hose connection a little forward of the heater-hose bypass looks like it is missing a hose clamp. That could be a problem.
From the residue around that area (the kind of white spots), I can tell you that is what coolant residue looks like.
I would clean everything up around there, replace that missing hose clamp, pay very close attention to the heater bypass kludge, and test again.
If ever in doubt, you can try putting a wadded-up paper towel under whatever you think might be leaking (and maybe tape it in place). Yes low tech, and yes, finding external leaks really should not be difficult.

 

First tool I ever bought. This, and an extendable magnet. I was 6, and used my allowance when all my friends were buying .05 candy bars.

Your bypass looks ok from what I can see. But whatever the small pipe in front of it is, (without the clamp) is where you're leaking. I'd say throw a clamp back on it and you should be good to go. Personally I like the gear clamps, unless you tighten them too much and it cuts the hose. But a spring clamp is fine on that one, seeing how that's what was there.