Silicone is massively inert in the chemical sense. It doesn't react to much of anything, which is good for aquarium purposes. Not so good for using chemicals to remove it since the silicone will go all passive-aggressive and say "meh" to about anything you use.
The way to remove it is mechanically, which is tiresome and time-consuming but works. Get a pack of single-edge razor blades and just scrape each wall--being very carefull around the corners of course, since you don't want to loosen what's holding the tank together--until the unwanted goop is gone. I would start on whichever side is going to be the back wall, in case you scrape so enthusiastically at first as to create scratches on the glass. At least that way you can always put in a backdrop, or in any event won't have to look through said scratches to view your fish.
by the time you get to the sides and walls you will have perfected your technique and it will come off smoothly without harm to the glass.
Mineral spirits are basically turpentine. Rinse, rinse, rinse with plain water to remove but most of it has probably evaporated already so you are just getting out any residue. By the time the tank has cycled for future use the rest will have have come off the glass into the water and evaporated off the top and you are good to go. I would not recommend bleach for this or any other purpose....well maybe if you have had a horrible BBA infestation or something but not otherwise. I am assuming that the tank is presently emptied after the re-siliconing so we are not going to be affecting any creatures or plants by any of this. I think you should be fine.
although I would not hire the same people who did this job for any future leaking-tank repairs. I am trying to figure out how silicone would get "smeared" all over anything? It just goes in the corners where the sheets of glass are held together after all. I guess we are all beginners at things at some points and maybe they will do better on future jobs. Best wishes to you and your fishes.
