On a realistic note, is there an official redline from China about Japan’s offensive military capabilities?

No. And the US wouldn’t take such a line seriously as it’s unlikely China would either.

In the past crossing China’s red lines meant being snubbed for diplomatic visits, some not picking up phone calls, reduced counter-narcotics coordination, and angry military drills around Taiwan and absolutely nothing else of serious consequence. The trade war has China tit for tatting the US with export restrictions but that’s not as a result of crossing red lines on geopolitical issues that displease China but as a direct response to economic attacks. The US has a history of crossing China’s red lines on Taiwan in particular. China draws one, US steps over it, China gets angry, acts coldly towards the US and eventually after a few years goes back to normal after some gesture move from the US on unrelated matters or some formal statement with no teeth.

Fact is Japan is a US vassal, a directly occupied vassal and has been since WW2 and a key part of US power projection in Asia. If it’s in the US interests to have Japan re-arm and say “we are doing this to mass murder Chinese people soon” then that’s what will happen. China may express displeasure by slapping on trade sanctions on Japan and allow and encourage protests in China against Japan and so on but they’re not going to strike Japan first because militarily striking Japan would be tantamount to militarily striking the US as they have a joint defense agreement. (Source: https://en.wikipedia.org/wiki/Treaty_of_Mutual_Cooperation_and_Security_between_the_United_States_and_Japan )

At this time in China’s long-game plans they have no interest in engaging in warfare with the US.