1) Don't know your bike but I would guess the chainstay will be too short to use large panniers (spec sheet for all but the xs says 405mm by comparison my Panorama as 450 mm and being a big footed git I can still get heelslap if I don't pack carefully)
2) Just returned from my first tour, 3 weeks solo through the south of France, I would say you are never alone, every time you stop any passing cyclist will enquire as to your well being. If you do join forces then on climbs and descents you will split up in anycase (I met 1 couple who where spending a year touring,they met at the top of climbs, at a coffee shop at the base and then again at the campsite!) BUT it can be nice to have company on an evening for a drink and a meal if you don't speak the language and end up in the back of beyond.
3) Mileage will depend a lot on your timescale and wether there is anything to do where you plan to stop/ find along the way, and wether you are alone,terrain etc etc. I planned on 80km a day but ended up doing 40km some days,140km others. By setting off in good time I was basically at the end of my planned day by lunch time,which gives plenty of options.
4) Plenty of campsites and wild camping is easy, I even pitched up on a village green one night without problems. I had a planned route but that was because I had a planned climb most days. I would say it is best to have a good map at least and a daily plan to avoid main roads. I have considered doing a similar trip next year and thought I would just generally head south
5) I used cleats with SPD shoes remember to keep checking they are tightened each night,particularly if you do much walking off the bike.
I didn't bother with the weight of a stove and fuel. I figured if I could buy food to cook then I could buy food to eat. This turned out good for me, Boulangerie for brekky, cafe for lunch, then a meal out at night or a baguette a cheese tomatoes fruit and a bottle. The couple I met on the year long exped. had sent their stove home after six weeks ,they said all they had used it for was a brew on a morning.