The fork mounting should be fine as long as you really never reach for it or put it back while riding. I'd put one bottle on the downtube as usual to use while riding and keep the fork mount only for carrying a spare that'll be exchanged while stopped. That position is about the same as low-rider panniers and normally doesn't cause any instability. I used to use a single pannier in the front for my commuting stuff (weighed much more than a full bottle) and never had any handling issues.