Oh, gotcha. Still, you may want to get a HM slave, not like you can relearn Scald at any point (not for free anyways). Grab a Bibarel, they learn all field moves except for Flash and Fly.
Well, keep in mind that only 2 Field effects activate that thing, the Grassy and Foresty one, which we only saw on and around Route 1 and Jasper/Beryl, where no important fights happened in which the defense boost would've helped.
I really don't think you need a rotation. I only ever had more than 6 members once and that was 8 just because I couldn't decide which 6 I liked most. If your main 6 are doing their job, stick with them. If you need some 7th to help, it'd be better to get the best possible something for that specific situation and use it only for that, cause your main 6 are good enough.
Well, Rotom really isn't that good since it's mostly used as a wall in comp, because that's what it does best. And it doesn't get it's favorite toys in-game, so it's really not that great. If you really want to use it, it's best option is a bulky Charge Beaming set, but I really wouldn't expect too much from that.
Well, you can with Gengar, but it's obviously veeeery frail. While Gengar could hit hard once and die the same turn if it doesn't OHKO, Cofagrigus could take way more hits and it's still got 95 Special Attack, that's nothing to scoff at. There's also the fact that a lot of your team members really aren't that fast, so a Trick Room setter would be very helpful. I'd go with Cofagrigus, make it Max HP, Nasty Plot, Shadow Ball and HP Fighting and watch it sweep.
Another option for the ultimate physical wall is Gourgeist, but it doesn't really fit into your team, so I'd still go Cofagrigus (still don't think you need a rotation, just swap Gardevoir for Cof).
Unless you're facing some wall, slowly chipping away at the foe's health percentage-wise isn't that useful at all. And even then, if it's a wall, it really can't do much to stop you from setting up in front of it and just overpowering it.
Yes you do. See, Egg Moves are passed on by both parents in E13 and onward, so DD is no problem, but Rock Slide is a TM, which can only be bred in E12 or prior, when only the father passes on moves, so you can't breed a Charmander with DD and a Charmander with Rock Slide for one that has both. Your only options are going through the pain of mining up a Helix fossil, then doing a series of side-quests to get Tyrunt, breeding DD onto that and getting it to be a level 68 Tyrantrum, then breeding that with a female Charmander/meleon/izard.
The best answer I can give you is "just keep playing" it's an area you have to go through, the earliest you'll get to it is when the plot needs you to, which is after your ninth badge.
Go with HP, it's not supposed to defend against a specific side of the spectrum. Also, the difference between only one defense or health is next to nothing. An Abomasnow with EQ does against a Max HP/0 Def Magnezone does up to 96.5% while a 0 HP/Max Def one takes up to 95.3%. The exception being stuff like Dragalge, which won't take physical hits well no matter what, so you might as well go with max Sp.Def, and things that boost one side of the spectrum, like your Gogoat.
No prob :]