”Hirohito and the Making of Modern Japan” has published in 2000. "The Memoir of Emperor Showa" by Mariko Terasaki Miller was written in 1990, a year after Emperor Showa's demise. Miller was an American citizen but her father was a diplomat/translator for the Emperor and GHQ, and he kept and left her numerous memos. The circumstance was credible enough but at the time, the source couldn't be confirmed. Bix seems to be based on the memoir anyway, considering when his book was published.

The 25th anniversary of the demise of Emperor Showa, Imperial Household Agency had released the complete "The chronicles of Emperor Showa" (昭和天皇実録）in 2014. In them, it does seem that Emperor Showa was against the war (Japanese soldiers and citizens in China been killed), as he was visibly upset that the military leaders didn't explain enough to him at the meetings, f.e in "Nekka Sakusen" (Operations in Rene?) in 1933, while Takeji Nara, then military attaché to Emperor, sent him a very threatening letter saying "Please don't slow down meetings, or we have to worry about coups...". Tsuyoshi Inukai, the prime minister, had been assassinated only a year before. You have to remember: Emperor Showa was still in his early 30's, Japanese military had won the Japan-Russo war and became the "guiding light" of the non-white civilization (described by people like Tagore and even Sun Yat-sen), and while Maresuke Nogi, the hero of the war, had killed himself along with the demise of Emperor Taisho, the military and navy was still very respected.

Did Emperor Hirohito realistically have an opportunity to double down on support of the ailing Taisho Democracy, and prevent a military takeover of the system?

I'm not sure what you are saying here. Historically speaking, the term "Taisho Democracy" is the part of the Meiji-Showa democracy, describing of the time Japan had won at WWI, being chosen as one of the permanent members of League of Nations, and issues like Labor/Feminism movements had spread from the west along with motion pictures and fashions. It's true that the assassination of Inukai is considered as the end of the Taisho democracy period, but it's not like the "civilian government" was taken over by "military government" as the war had spread, as even Hideki Tojo was forced to resign in 1944 by vote majority in the diet. Throughout the Meiji, Taisho, and early Showa era, there were fair shares of civilians and military officers being ministers. I believe it's harder to create dictator and dictator government under the parliamentary democracy systems.