1. i can say yr condition is NOT BAD, as u said ur ACL torn for a yr and had ONLY ONE episode of instability while playing football, right?
2. For football, lots of pivoting, sudden stop, etc, which is challenging the knee stability (ACL). While for kitesurfing, basically, it's much less loading! Unless u're as radical as Len10, for just riding/cruising, simple jump, spin/loop, etc, I BELIEVE U WILL BE OK!!! But if u try new move that is more radical, then, it's another story!
3. Surgery or not? GOLDEN RULE: SEVERITY OF SYMPTOMS & THE NEED!!!!!!!!!Yes, if u have much recurrent & persistent symptoms! And if u're young, say under 35', and u're so desparate for the football & Tennis. Otherwise, if i were u, i would MODIFY MY LIFESTYLE, DO CYCLING, x-country is good fun too, cycling is LOW IMPACT exercise, and not much pivoting, and it STRENGTHEN YR QUADS, HAMS, and calf EFFECTIVELY (esp if u use locked paddle...). U're not Rolnado who the football is his profession, right?
4. For the reha, i'm not sure the standard of yr physio, try to find one good (Sports) Physio! Quad is important, BUT, You have to actually further strengthen yr hamstring & gastro, as they help to pull yr lower leg backward (which ACL does), proprioceptive & agility training is important too!
5. Risk of arthritis? This is the excuse of someone persuade u to do the surgery (sorry Ortho Doc), this may be true if u keep football/tennis....but if u follow my advice above, i don't think so...WHY NOT TRY & SEE!!! And just let u know if u don't have much swelling/pain/give-way feeling in the future, u're fine!
To conclude, yr condition now is gd for kitesurfing, mate. Try not to do football & tennis, modify to cycling for the no-wind days! Cycling training is amazing, do interval training > 1 hr, x3 per week.