No need to disconnect fuel line.
With it disconnected there's a good chance fuel might go where you don't want it.
As others said, there's something wrong with the gauge or maybe it wasn't screwed in enough.
Even with the short video you can see the temp gauge go up slightly.
You could disconnect the wire at the temp alarm sender and see if the alarm goes off just over 170.
If your motor is a 1998 it should be covered in Mercruiser Service Manual Number 10, Publication Number 90-14693-1
Ebay has a number listed, some of which show which engine serial numbers they apply to.
Search for Mercruiser 90-14693-1
Mine never goes over 140 idling on muffs and that could take 15-20 minutes to get there if I did it that long.
I normally have the throttle advanced slightly on muffs so it might be a bit higher than idle speed but not over 800.
As far as I know, that drive calls for the High Performance gear oil, not Premium.
If I can't get the High Performance for a good price I've been using Synthetic 75W90 for a number of years based on Bondo's recommendation.
You won't feel power touching battery + and hull.
Just like you won't if you touched the + and - terminals on the battery.
It has to overcome the resistance in your skin.
In theory it's possible, but unlikely.
A friend has a 16 ft Lund SSV utility boat.
3 bench seats, nothing else.
boat=345 lbs, 2 guys=300 lbs, 15 lb anchor and 3 gallons of fuel.
With a 25hp Honda we get 25 mph.
My bad, I measured between centers which isn't going to help you.
I just measured between the studs on an old MR/R bell housing and it's 1.50 between the studs.
I had the bell housing off when I pulled mine though.
The claw pivots so you can insert it.
https://www.summitracing.com/parts/wmr-w89326?seid=srese1&ppckw=pmax-tools&gclid=EAIaIQobChMIx-OztdjqiAMVUTcIBR0myiwAEAQYAiABEgLA-vD_BwE
