merc 5.7, low idle, stalling, milky oil in filler neck but not in oil pan

[jamanati]

Have a 95 3200scr, got it about a month ago. I dont know the history of maintenance. It has twin 5.7 with bravo 2. I initially changed oil, filter, cap, plugs, rotor, fuel filter. Been running okay. Today, dropping it into idle while coming into the marina, port engine stalled. Started it several times, I noticed idle was very low, around 300 rpm, compared to the stbd which is about 650. Putting it into gear would stall. I started it again, it revs up fine, but idles low, around 300. Water temp is fine. Got it into the slipt, and into the bilge I went.
I visually checked the carb, nothing really stood out. I noticed a yellow-ee substance on the valve cover flange. So I unscrewed the oil filler cap and its frothy white a bit. My blood pressure went up at this point. I checked with my mechanic he said to check the oil in the pan via the dipstick, which I did and that oil is fine, nice and clear since it was recently replaced only a few weeks ago, so there is no frothy white stuff in the oil pan. He said it may be leaking risers, to pull some plugs and check them. This may explain why it was idling low, maybe some water is getting onto a few plugs. I haven't done that yet, planning on it tomorrow morning. But a couple of questions to anybody in this forum that can help.
1) Mechanic said that sometimes the oil gets frothy because of some condensation and simply from the valves going up and down. First time I heard this. Anyone ever see/hear anything like this and is it something to worry about?
2) These are 5.7 mercs, anyone know the torque specs for manifolds and risers? I heard they have to be re-torqued after they get hot?
3) Any tips/tricks to replacing these? I already got one, when the first 2 bolts from the manifold are pulled, replace them with long studs, about an inch longer. This way when the other 2 bolts come out, the manifold doesn't fall or need someone to hold it. Then you can just slide it off the studs. And when putting back the new gasket and manifold, you can slide them back onto the posts. Any other tips?
4) Mechanic mentioned something about an oil blow-by? What is that?
Thanks all.