Two things,
One, I NEVER suggest extract in a beer this big, you cannot control the fermentability of the extract, that is unless you added it to the mash. How much did you add?
Two, a VERY long mash. IMHO 75 min is not enough, at least 2 hours.
OK, three, you mentioned the "Rocket fuel" word, Did you say this because the alcohol is HOT (higher alcohols, fusels), or because it is just higher in alcohol?
Beers of this size MUST be fermented cool, wort temps in the 60s, the cooler the better. If it is a hot alcohol taste get a growler FULL of yeast from you local brewpub, any clean yeast will do, but this yeast is FRESH and ACTIVE, let it settle and pour off most of the beer leaving just enough to allow you to pour the yeast into the strong scotch ale. The active yeast will help some, but time in a bottle conditioned beer will help minimize the heat.
I suspect that you are done. The body of this beer is supposed to be really full.
You will not get to a FG of 1.020, as your lhbs suggests unless you make your 5 gal of beer 10 gallons of beer, in a word, don't.
That said I think you did a fine job!!!
From Greg Noonan's book, Classic Beer Styles Series on Scotch Ale
1850 140 Shilling Edinburgh Ale, OG 1.125 FG 1.055 and no, I didn't mistype that
A double mash recipe 140 Shilling Edinburgh Ale, OG 1.125 FG 1.055 (Second beer Twopenny 1.040 / 1.013)
I have brewed some awesome beers that have an FG of 1.065, I was told by many, "don't you DARE throw it out".
Bottom line, bottle it and enjoy the fruits of what you brewed. Remember how you got there and use that knowledge in your future beers.
You done good, bottle it and enjoy it for what it is.
Fred