Quintonboy1

Anyhow let's make an attempt to properly answer this question. 
1) it is not unreasonable that a player can spend 10+ hours grinding away in the game. I've seen people go for much longer in my time in vortex, (7+ years experience). 
2)The maximum amount of battles one can achieve statistically in 1 hour is 360 battles, this takes the 10 second battle timer into account. Meaning in 10 hours the peak performance is capped at roughly 3,600 battles. Considering 600-700 is much smaller than that it can be achieved in a much smaller time.
 
Now I know seeing numbers like this is going to be practically impossible as only a machine can maintain such consistency over a 10 hour period of time. However even if we adjust to a mere 1 battle every 14 seconds the goal is still very much achieved in a reasonable time, with room to spare for catching to be involved. As well as a few short breaks in between. 
        There have been many methods born through trial and error in this game to improve performance when it comes to the battling aspect of pokemon vortex. With a combination of this and experience there are many who can achieve what you deem to be impossible. Who knows with enough knowledge and dedication put into the game maybe you can achieve the "impossible" as well.

As far as I'm currently aware there is no direct rule stating that this isn't allowed. Two people cannot be logged onto the same account at once however. 
 
Overall it can seem like a good idea, but you give up your password and put your account at risk in many ways. For example if the person your sharing an account with gets banned for malicious activity such as botting on his main account, it is possible any accounts they logged into could recieve a ban as well. There is always related scenarios that can potentially happen. I'm not going to say that everyone is going to break the rules at some point, but do you trust the individual not to be tempted to fudge some rules let's say 2 or maybe 3 years down the line?
          I only approach this question with this answer because I have been playing for a long time and have seen stupid stuff happen in the name of trust. Be it scamming, or banning it's always a possibility. 

As far as i understand it there is actually no reward apart from bragging rights. Its been that way for a while, and i'm not to sure if it will change anytime soon but to me it is unlikely. If you want to know why you should do clan battles i would say probably to unify your clan, work towards a common goal, and set "in-house" rewards for your members. Its basically another way to have fun with your members, may not apply for all, but for the members of AntiWorks thats how we utilized the clan battle systems.