Stoaty-
As far as loading the speargun, I actually put the butt of the gun in my hip (more comfortable)
I would not shoot the gun out of the water again, Just not safe.
You probably don't need to replace the band(s) but I always carry back up with me "Just in case" one were to break.
I'm guessing you will get cold in that wetsuit- The best suits to dive in is "Open Cell" on the inside of the suit. Search the forums for "Freediving wetsuits" My recommandations to my friends is always "5mm open cell - Elios" But that's just me
Your fins - when you say "diver" you mean Scuba? which if that were the case, I'm guessing that they are maybe 15 - 18" long and really flexible. They "WORK" but you'll have much better luck out of a freediving fin.
Just my 2 cents (or what ever that translates in Euro's) I'm sure others will Chime in- Good Luck Spearing!!!!!