I was thinking that maybe Tatsuya could get better with Regrowth if he could cast it on like 10 people around him in auto-restoration mode just like it's the case with him so that once one of them is injured they get healed immediately without him having to look after them(if these 10 people are elites combined with tatsuya's abilities they could take on hundreds/thousands of foes).
I doubt it can work that way. There are multiple reasons why.
1) If all of them take damage at the same time, he'll suffer from a huge amount of pain at the same time if it automatically does that on all of them together.
2) He'll need to constantly monitor their body's condition, taking away large amount of his attention, which he can and should be using against threats in a combat situation rather than spend all his time "baby-sitting" people around him constantly (he already and only does that for Miyuki).
3) Each cast of his Regrowth ability requires huge amount of information input, which means he will need to keep a large amount of "backup" data from the constant monitoring of the "friendlies" around him that it will impact his combat abilities. You have to understand that, from what I have read about Regrowth, he need to create a "storage" to contain all the information of his target (if I remember correctly, this is something Miyuki sensed when he used Regrowth in the Recollection LN08, else it was something in LN07, I'm just remembering offhand) before he cast Regrowth, and this means that if he has to do it on a "auto-restore" mode for a group of people around him, thats a huge amount of data which he has to hold and this will impact his magic abilities.
Replied: March 04, 2013, 10:31:18 PM
Yeah she saw him use Decompsition Mist Dispersion on a large target and now knows it belongs to secret JSDF stuff. Knowing what its called and how he did it can be summed up as he used his secret magic and left at that.
MB is still decomposition magic and knowing he can take such magic to such a level should't really change much for his friends as they already know he is a secret weapon for the JSDF and can use powerful offensive magic.
And anyone actually close enough to personally see or sense Tatsuya's Decomposition magic trigger a nuclear explosion would actually die in said explosion. Its something you would have to use monitoring equipment to catch no weapon of science was used and then theorize what magic was used or Tatsuya himself would have to explain he did it ad how.
Strictly speaking, Mayumi only know that he "did something" but not what he did. She doesn't know the extend of his magic ability, only the end result of that target turning into basic element particles.
MB while used by Tatsuya, is not known to his friends/classmates that he is the one who used it. They might suspect it but it is not something that they know for sure. How many of them could actually relate it to Tatsuya is still unknown. Mayumi might be the only one who can put 1 and 1 together, however since she didn't understand what she "saw", it is still up in the air as to how much she is actually speculating about it (since it was never mentioned at all).
About your last point, it is not really something that was actually monitored when Tatsuya used MB. How the International Magic Association/Organisation judged that, nuclear radiation fallout causing device and magic
were not used in that attack, was never explained; only Maya saying that they have reviewed and judged that it was not, and thus no sanction will be passed on Japan.