I would have to agree with Jerry. I think there is too much being made of the sludge. It doesn't look like a coolant leak at all to me. Looks more like the sludge that is created form too many short trips and not getting the engine to temp. I've seen hundreds of valve covers over the years that look like that and it had nothing at all to do with a coolant leak. That's probably why it never appeared on the dipstick.
Oil is lighter than coolant and if you have enough of a leak to get that on the valve covers you should have a bunch in the bottom of the pan when you drain it. If you've already pulled the intake, you may have got some in there, but if you drain it before removing the intake, there is probably little in there. Condensation is always present inside and engine because of the exchange of air from the cooler outside into the hot interior of the engine. Getting it up to temps is what evaporates it. Most vehicles that you see with that type of valve cover sludge are usually used for short trips...maybe back and forth to work and never get completely up to temperature. That can kill a motor as bad as a coolant leak. IMHO.