Hey there, I will try to answer as best as I can but some of these questions don't have a 100% way.
1) /sam, /war and /nin are all good options for exp/solo/NM. It all depends on your survivability and play style. I am not so sure about /sam because I never tried it, but I remember people that played healer type like this sub job. The war sub job are for those that want to increase their attack when they are fighting things that have high defense and /nin is obviously for survivability. As you get to higher level and your gear becomes really good and you are able to get merits things may change for you. The ninja sub job is the most used by people though but that doesn't mean it's always the best. It's just a safe bet for a lot of the times.
2) Use whichever you feel you need more. You know if you need acc or attack. I try to improve accuracy for the party since most people don't eat food anyways so I'd rather they hit the mob. Also, fights shouldn't last long enough for you to keep alternating quick/box step in a fight that much. If they are overhunting that much either move or make a new party.
3) At lower levels, I prefer rice dumplings as they give an acc and an attack boost. The food is only 30 mins long so unless the party disbands within the 10 mins, you shouldn't have wasted much. When it gets to birds, yeah I'd go with cheap spammable food such as they ones you mentioned. At later levels, marinara pizza becomes a great option for an acc/att boost.
4) Those are great options for atmas for general exp parties. You might want to invest on some evasion atmas, magic defense, regain, acc and attack atmas. The Tier 3 VNM in Tahrongi Canyon has a great atma that gives both attack and evasion. The tonberry NM (can't remember the name) has one that gives acc and evasion. There are tons of options and it wouldn't hurt you to go after the good ones even though you may not use them that much.
5) I can't help you much with this but I always loved bringing out my npc fellow and going trough EM mobs non stop. You don't get as much exp per kill but he helps speed up the fights and take away some of the soloing boredom. When I leveled my dancer, there wasn't abyssea so I didn't get to solo much. Sorry.
6) It's a great piece of gear if you don't need the accuracy. Well it used to be a great piece of gear until that mini addon came along and released a piece that can be augmented on par/better than Rap. Harness (This was dependent on amount of haste you have) and the now empyrean body armor. The thing is that you will probably get a party at level 70 for abyssea and on that same night be level 90. So, yeah you could use it as a hold if you don't have anything better. It is also a good piece of gear for casting shadows as it is the only body piece that gives highest amount of haste for dancer (not sure since abyssea).
7 & 8) This is the simplest form I can explain this. No form of magic that improves your speed will reduce your tp. Your tp is based on the delay of your weapon which is only affected by dual wield. So, for question 8, yes dual wield will reduce your tp.
on this site you will find a better explanation and it is very important that you read about it so that you may understand how much better is is for you. http://wiki.ffxiclopedia.org/wiki/Delay
Keep in mind that the faster you are attacking, the more tp you will gain. I wouldn't worry about them affecting you negatively which they shouldn't be when you are doing it right. You actually want a lot of haste (gear cap is 26%) and dual wield with weapons that benefit you.
obs. Most people say 25% is haste cap for gear but due to how the game calculates things, at 25% you are not capped. However, at 26% you are over the cap or capped. There is no piece of gear that offers you a fraction of 1% so you need 26% to be capped.
Hope that helped