In case you don't have an operator's manual, I have posted a link (since the file is too large to attach). It includes troubleshooting info, as well.
file:///C:/Users/USER/Downloads/7744286.pdf
The fuse block should have a fuse specifically for the injectors and ECM- you might also remove the plastic cover from the top of the engine and post a photo- it will be easy to tell what it has for injectors and the manual has several models and yours is included.
I have serviced some Volvo Penta marine engines and the more recent ones were solid and seemed well-built.
A fuel injected engine should start easily without advancing the throttle. As I posted, one fairly common cause for the need to open the throttle is a bad ECT sensor. Aside from that, there aren't many reasons to do this.
Does the exhaust smell strongly of petrol? That's an indication of the engine running rich.
If you live in a place where the temperature drops below freezing for long periods, I would look for a painted plug on the front of the intake manifold that has a square head- if this isn't removed, water can remain after draining the block and cooling system, causing cracking due to the water expanding. This was pointed out to me by the shop manager when I was about to winterize a V-P engine for the first time in years- I had never heard about that plug and it requires a certain socket, but it needs to be removed. It shouldn't be difficult to find- IIRC, it's a 5/8" 8 point socket.
Also, some tricks are involved in replacing the impeller and OEM parts should be used (inclucing the gaskets and packet of Glycerine, for lubricating the rubber).