I have a 115 hp 2+2 Mercury 2 stroke. It is basically 2 alternate firing twins on one crankshaft 90 degrees out of phase. It's basically a 57 ½ HP trolling engine and a 115 get up and go. At lower rpms the top two alternate firing cylinders generate thrust and the bottom two plugs fire, like the top two, and get fuel, but not enough to generate combustion. As a result the plugs sit there cold, getting wet with fuel mix. What this does is tends to foul the bottom two and if not too fouled, they will kick in when the throttle it advanced, but it takes awhile to clean up and be able to run at full rpm.
In looking at the problem and the plugs, I saw that they weren't burning off contaminants when they were running. Since I do mostly midrange or slower running, like you, I am not worried about getting too hot a plug. The other thing is that I wanted to increase the surface distance that spark leakage had to travel. Finally, having had a career in high voltage and capacitor discharge systems, I know that the more concentrated the area of the spark, the more intense the plasma.....sharp points are easier to light off.
I went to the NGK online catalog and pulled up my service manual listed NGK plug for my engine. It was heat range 8. I dropped it to heat range 6, lower since NGK runs hotter with lower numbers unlike most popular brands that do just the opposite. Then I went to the Iridium section with my other dimensions and found an iridium plug that matched my OEM plug dimensions.
They have been in now for two outings and upon removing and having a look, the insulators are nice and tan as they should be and all 4 plugs came out DRY, not top two grey and dryish and bottom two grey and wet. The other thing I did that was a immediately detectable improvement, was to tilt the engine up in the rear, like 10-15* ANY time I am off plane, which keeps fuel from puddling around the spark area.....I have posted several times on this and referred to early Mercury smaller engines deliberately built to run with this tilt for this reason.
So that is what I have done to correct my engine to suit me and I have no plans to change anything. If I still had a boat full of kids, and if Sunday afternoons were spent water skiing and lots of high speed running and all that then I wouldn't have done what I did. But those days are past, my boating has changed, as have my spark plugs and driving habits.