No, they never said who’s plan it was (until we found out it was Mueh’zala) but they never told us that Sylvanas knew about it.
Yeah it seems like she was unaware of it, and it was something Muezhala sprung on her at the moment, and she just went with it. Especially in Before the Storm, when she curses Voljins Loa for putting her in the position. But who knows anymore.
Yeah, at this point, with the Soul Fragment shenanigans they are gonna say she knew all along even though she felt bad about it, she went along with it anyway.
Counter-argument:
Garrosh’s plan was to engage the defenders of Theramore (with him leading personally) without intent to breach; just to harry and intimidate. In this time most noncombartants were evacuated from the city.
leaving just, you know, the most powerful arcane spellcasters in the Alliance, a retinue of armed defenders, and, you know… the Blue Dragon Aspect himself.
Honestly sending dudes with swords against just Jaina is a perfect waste of Horde Lives. neverminds all the rest of the firepower that Theramore had at its reserve.
So blowing the place up with an arcane bomb made perfectly good sense.
Jaina of course has an emotional meltdown because her pet gnome died… One wonders exactly what Jaina thought would happen when she decided to add Theramore to the Alliance’s war effort.