I will address these two questions
Regarding MM2 as a bad match up, I wouldn't argue it is bad on its face. I would argue that it is bad because the MM2 player only needs to get one MM up and running quickly + 1 Lysandre and PRC Gardevoir is probably behind already. Allow me to pose this scenario as an example: Say MM2 goes 1st. Say you even start perfectly and get Xerneas up top, 2 Gardevoir EXs on your bench, 2 spirit links on your Gardevoirs. Let's even make this super awesome and say that at the end of T1, you have 1 Fairy energy in your discard + 2 BA Gards in hand + 1 Mega Turbo. Because you went second, both BA Garbs start with 1 Fairy each + 1 Fairy on Xerneas. T2, MM hits the board, they Mega Turbo into 1 Psychic + 1 DCE on the MM. Let's just assume they added the other DCE to a benched M2 to set up the second one rather than just stomping your Gardevoir T2. They Lysandre up one of the Gards and hit it for 130. T2, you level into BA, attach one energy, use Mega Turbo and get another onto BA and for kicks, let's just say you swap them out free with Fairy Garden. Now you have a 3 energy BA waiting to hit MM2 for 150 (5 energy on the board). That's solid. Except that now you are absolutely relying on heals + 0 energy attachments on the current active MM to survive the next hit. All this while having a Gard on your bench that has been damaged for 120.
Basically, what that book of a paragraph is attempting to establish isn't some crazy scenario. That is common for both you and MM2. The difference is that MM2 is one energy connection away from burning your active, is setting up a second MM free of damage, and only has to hit your eventual BA Garb for 90 to knock it out. You will be down 4 prizes to 2 after T3. That's hard to overcome. Not impossible. But hard.
By contrast, DR Gardevoir ends the threat T2 by knocking out MM #1 and forcing MM #2 to have a whopping 2 DCE + 1 P to knock out your DR Gardevoir. That absolutely turns the scenario on its head...all by using the options available to you by having two different primary attackers.
As for the DR Gardy "mirror", I honestly don't know that I think it is any better or worse. Both need to hit for 210 to bring down the mega. DR Gardy can get two up and running faster with fewer turns needed to make that happen. By contrast, BA can be up and running on a full 1 shot DR with only 7 energy on the board (which is extremely likely by T3 and quite doable by T2 if you can get 2 Geomancy off / hit enough Turbos). It always turns on whether BA Gardy gets heals and DR Gardy can keep that bench up. If BA can limit the bench to 5 with the right stadium, BA should win with enough healing because BA 1 shots DR with 7 energy on the board. Dunno. I think the match up is pretty even IMO. Just depends on who gets what breaks.
I am sure others have different experiences with that match up, but I haven't found BA to get overwhelmed by DR H2H.