1. I'm obviously biased here but the answer is yes You don't have to take my word for it though, there's people switching from that software to ISBoxer all the time, including some of the moderators on the other site. That other software seems to be less functional than Keyclone to be honest, other than buttons you can click with your mouse (which I'm told not many people use). Here's a guide that can help you make the switch -- and the Quick Start Guide is a more general getting started guide that has a lot of good info as well.

2. There's always pros and cons, the cons being that it might be harder and it's extra $$ to buy the game, expansions and the subscription. The more characters you play yourself, the more content you're going to be able to complete on your own. (up to a point) You also get a higher chance of getting loot if you're playing with other people, etc.

3. Pick your favorite DPS. The main question is whether you want melee or ranged, and then what utility/buffs you want. It doesn't really matter Shaman, ret paladin, and mage seem like pretty good/common choices

4. "Boost". Use Refer a Friend to add any new accounts, then you can power through 1-60. Do the referral from the prot paladin account, then you can run the new character through dungeons with him (can pretty much just put the lowbie on autofollow and aoe)

5. Up to you, I probably wouldn't.