To my knowledge I'll answer some of these. Take note that I'm answering these from my own experience and knowledge so it will perhaps differ from what others will say. You also do not need to follow these if you do not want to.

1 -
It depends on your server. Since this game is not as active as it once was there are not many customers and also since newer players do not have the benefits that older player have some players choose to be helpful and give them a lower price. You can always still put "S> DS, RB, JS" etc... in your nickname so other can see that you are a seller. Also keep an eye out on world to see who buys. Make certain that you are able to do it and are confident in being able to pull the quest off first before you think of selling it.

Also have to take into consideration that some people will look at your level and may not want your help some will even have a rude way of showing it. Unless you are well known In the game and around your server.

RANT

2a -
I think it should take somewhere between 2-4 months

Non stop online, unless there is maintenance or your need to turn off your computer to give it a break. Also if you use pots and what pots you use. You can also get 55k exp daily from instance; Sealed, Blue, Copper, Fiery. That may help as well. EXP will slow down the higher you get since the exp bar gets longer. By the time you are 110+ it will take about 1 week per lvl. Also if you have pots use it every weekend. Pots are more effective on double exp days. So do the instance that give you pots. Besides 15 round you can also do NT for chances to get pots and capsules from eggs. CP will also give you a 4x. Best to only do it on the weekend so you save yourself stars and not do it on the weekday. Registration vouchers are free from NT and Q&A

2b -
Reason you train in groups is because more damage means more exp and most important of all your water can auto heal and revive. Unless you have the money or resources to cook food everyday to sustain a single player then go right ahead. But I see no reason to train alone when you have a water you can use. Water obviously need an e-remote. So if you do not have an e-remote or even a water then sure go ahead and train alone. Also only time I would think you should train alone is if you are at Training Island.

3 -
Reason why most people train is Rome; Not everyone has access to sharks and getting the key is just about as hard as getting FB. Its easier than FB but harder than DS.

You can train at sharks at any lvl as long as you have the HP. Best to have 900 (lowest) or 1200 HP at least so you do not spawn. Also take into consideration that if all 4 sharks combo on you its likely to be 1200 to 1600 hp damage. The more defence and hp you have the less damage you take. Sharks are better because they are highest lvl in game (at least until an update that creates even higher mobs). Obviously higher the mob more exp. Would you train at chickens with your 100+ team?

If you want to train with gear on you will need to be lvl 132.
At Rome gear is wear able at lvl 106.
There is also the seahorse area above sharks, the gear is wearable at 121.
The stingray area before the shark door, gear Is wear able at 120 be careful cause sometimes a random shark appears.

4 -
Earths are usually better when it comes to quests like DS and JS and even CP that reason being because of their stonewall. If you happen to have their FS that's even better. It shields you 50% in both physical and magical damage.

Wind helps if they are knights with knight JS to help anti-seal. If you have a pestle that's good too its your money. But I must say this DO NOT waste money buying Attack Pestle. THEY ARE USELESS and A WASTE OF MONEY. (I learned that the hard way) You will constantly need to repair it over time. They are like guns they break no matter what you do. I heard somewhere that you get 250 pokes with it. I'm not entirely sure, but with me when I used it 2 times it was already at useable. Either way its just useless so spend your money on other things. Maybe potatoes are cheaper but you can get some free in game from quests.

Also note that Attack Pestle has nothing to do with how much your attack is its about how much scrolls they have in them. The higher grade it is the more they take off. Wasting strong scrolls on something that's already useless is just stupidity. Its like using scrolls on pet's friendship brooches when all it does it gives you 10hp per upgrade. USELESS! (but using scroll can repair an item just though I'd point that out)

5 -
DS mob speed
2.2k soldiers
4.2k dragons
1.2k guards

JS mob speed
1.2k soldiers
3k dragons

Always best to be an extra +10 or even a +100 above to guarantee out speeding

6 -
Use any pet you want I say as long as they are useful. Do they help me seal? Do they help me with damage? Are they able to give me an extra hand with healing or defence? etc...
Useless pets are; Do they die after 1 or 2 hits? are they just there to be a meat shield? etc...

So if you have a pet you think that can help you seal, heal, use feathers, shield, stone wall, attack, w/e then use is. So fast answer; yes, pets like shasha, lan, qlaya even a monkey with 5k hp is useful as long as they can help you. If you have the money and can pill out pets like Fred to do mega damage then use it too. After all why waste money on pet food if your pet will just be sitting at the side drinking lemonade.

7 -
Do your pets do more damage than you the player? Then go right ahead and do so if you think that will help you. If you happen to have characters that are just as damaging I see no reason why only your pet should be the attackers often goes faster if you attack too.

If you have characters like waters and earths mages that mostly only seals then I think its better to have damager pets on them. And if you have damagers like fires then the choice is yours what type of pets you use; pets that help seal or pets that help damage if you hotfire them. (so attackers are often best paired with defence pets and defenders are best pairs with attack pets)

8 -
2k+ MATK / ATK
3k+ HP

So unless you plan to pill them out and spend money on food and brooch then go ahead.

Pets are just as useful even without pet food if you have a brooch and good gear. If you have no brooch then make sure they have good build and gear. So when lvling them make sure you sort them out at lvl 20 before they add stats to the wrong area. Some good pets who are able to do fine damage without food and brooch can be victoria, fred, clive, hayate, roca and maybe even sam, kanako or Maggie.

I know its a lot to read but I hope these responses would be of use to you. wow.. took me 2 hours to write this o.O I read your question at 5:52 and its already 8:16pm (had to spell check too)