Well, the blocked throttle is at least half of your problem. Fix it and try again.
The leak from the antisypon valve is typical. Pull it off and soak it in Rydlyme or CLR to clean it up. It has nothing to do with your exhaust elbow
Your inpection of the exhaust elbow
is interesting. I guess if you had seen dozens of them in various stages of fouling it would be telling. A year ago I had a Yanmar mechanic
do the same thing and pronounced it ok. I still would be suspicous however.
Puff of smoke on acceleration is perfectly normal. The fuel
injector is pushing more fuel
than can be burned cleanly to try to accelerate.
I do think that although you may have found most of your problem, fixing the throttle may not entirely solve it. Look at the prop since it is not oem.