They don't really have to enter the ground, one possible application is to use ES, link it to Miyuki then Cocytus bomb it (since he considered Material Burst then it's possible to target it remotely). Also, If Tatsuya has no mean to infiltrate the parasite location he wouldn't attempt to when pixy located them and Miyuki wouldn't vehemently stop him.
What I actually meant is that there was a scene pointing out they had trouble accessing the grounds undetected. Any actions infiltrating the event, releasing sealed magic, using powerful magic or even using ES could be detected and tracked as seen in previous chapters. Tatsuya also has to take a good deal of time to locate distant targets without certain aids.
I feel Miyuki's full powers being exposed by her blasting an entire park with magic, is a greater risk than the possibility of Tasuya's stealth field failing, visor shattering and magic failing to immediately activate to fix the damage.
I would have believed you IF not for the case where she accompanied him to various location during Lina and Parasite incident, her face is plastered all over the military ops now. Even Kudou already make the connection.
The military I remember monitoring Miyuki were the 101, STARS(even beofre the vamp incident), the unit sent by Koichi(who didn't even know about the dets) and the unit Retsu sent(who were nearly all massacred). All were tasked with covert missions, why would they reveal their secret jobs and plaster Miyuki's face all over? And why would Fujibayashi or the Yotsuba let them?
I doubt the Kudo researchers developing and monitoring the dolls or the JSDF unit in charge of the 9SC grounds were involved or know anything about the vamp fights. Retsu certainly didn't tell his son or the doll researchers about his plan to provoke Tats into fighting the dolls.
They still don't know Makeishura's identity, that's the risk I was talking about, not the involvement.
I know, what I tried to explain is that that is a 101 secret the was put in at risk with the ok and silent ok by 101 members. Tatsuya and the Yotsuba care more about Miyuki, not Tatsuya's military secrets.