Buffing AD bruiser makes her be flex top and jg and she's difficult to balance from there, don't you think? I used to main her when she was strong AD, but hate her AP playstyle. I don't think I agree that AD bruiser is stronger right now.
She could be. From what I can tell, SRT (Summoner's Rift Team) is pretty happy to say that flex picks are only problems when they become problems. See their buffs to Mordekaiser jungle and nerfs to Morgana jungle. I think they'd be happy with Shyvana being viable top/jg but would take issue if she became really difficult to play around due to flexibility.
As for her builds, looking at public data, Shyvana has about a 52% winrate when she completes Nashor's or Night Harvester as first item and a 52-54% winrate when finishing BotRK, Sunderer, Stridebreaker, or Trinity Force first item. Even Iceborne Gauntlet appears to be a possible strong starter.
Her picks are overwhelmingly AP (83% of games finish NT/NH first), but she still appears to be slightly more successful with AD fighter builds.