True enough, but getting overly defensive of it would be a massive weakness that Greymon X could exploit easily. His ranged attacks also all have decent AoE so in their fight, which should be in mid-range for most of it until she closes the distance, he shouldn't have too much trouble getting a hit or two in. Though, as I said before, just that shouldn't destroy her mind.
I wish I knew more about how their bodies worked, then I might know how exactly the data affecting ability would interact with their unique physiology. Are their minds stored entirely in the gems? Are the minds in the gems the only copies or are they backups for when the body is destroyed? Can the mind data be moved between the gem and hardlight projection? Her ability to astral project suggests data stored in this manner can be moved, but would destroying the projection in that situation be akin to destroying the gem...It's a little interesting, but somehow I doubt the answers are all in the show so I can't know for sure.
Using the astral projection as a starting point, that ability at least implies that mind data stored in gems can be moved or copied to another place, likely at risk of said data. However, if nobody else in the verse could interact with it they'd likely be invincible in such a state to anyone they might try interacting with. In such a case that she tried to enter Greymon's mind, he might actually be capable of destroying the projected mind data...but that's all theoretical. I happen to like thinking about mechanics in Digimon, because they can be so interesting.
Ignore this, it's just my rambling. Lore states that a MetalGreymon is a Greymon that upgraded it's body with robotic parts stolen from Andromon, but the greater size would imply it's an older Greymon, and there was actually an instance of that happening in V-tamer with a Greymon trashed by Neo. It returned after upgrading itself to grow stronger and appeared to be of average Greymon size, which would further imply it didn't take the time necessary to grow to that level naturally. However, in the shows and games that never occurs. A digimon recieves enough data to reach a higher stage and digivolves, ususally after either a long amount of time or getting help from humans providing them with Digisoul. In such a case we see the metal parts appear immediately and the size change happens at the same time. Using the lore as a base, it seems to mean that the changes that happen to previous generations are stored as data for that being to reach with less effort, as though they were getting a NG+ bonus after dying and being reverted to a Digitama. I used to love overthinking this kind of stuff when I was younger.
Sorry about that wall of text, it's not too large but I got a little carried away. I could go on longer but I'll spare you all the bother, I just had to get that one out.
And I'm still not sure who wins, but I'm starting to lean back towards Rose. I don't know, it's kinda jumbled in my head right now so I'm gonna take time to think it over. I'd prefer to be absolutely sure before making a vote.