The biggest scare with Seafoam in the intake is possibly pouring in too much at once and hydro-locking the engine (getting non-compressable liquid in the combustion chamber and bending a rod). However, I think this is pretty difficult if you take about two minutes or so to pour the whole bottle in.
Thru the main intake butterfly is not recommended for MB's because the manifold dips down inside, and the liquid will just sit down there and congeal with all the goo that's already down there.
The brake booster line is so large that keeping the engine running with that line open is difficult.
Best way may be to pull one of the small vacuum lines off the manifold, like the tranny vacuum line, and connect a hose/funnel there. Then just run the engine at fast idle and pour the SF in. The small line will restrict the volume, so you can't over-do the pouring. As soon as it gets nearly all in, just kill the engine - maybe not quite as effective as drowning the engine with SF liquid to kill it, but much safer in my opinion.

Let it sit overnight, then drive it pretty fast for 10 miles or so. should smoke like a demon. Then change the oil and filter.

DG