pre-mix might be the ticket to solve this but I would look into the things that control the mixture as its putting too much oil in so you might have a dirty or faulty sensor somewhere.
I don't know much about the oil injection systems so whatever is used to tell it how much oil you need is where I think your problem is.
you should make a regular habit of running it wide open for 5 or 10 minutes on the way in to burn the cylinders clean because the problem may just be that you aren't opening it up enough to burn the oil.
as to the hotter plug, yes if you just grab any plug that has a hotter rating you can cause issues but if you get the dealer or qualified outboard mechanic to recommend a plug that will only run "slightly" hotter then stock, and you monitor your plug condition it will tell you if you have the correct one or not. you cant just put a hotter plug with no thought to checking it to make sure its running correctly.
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