They will go to Avernus provided the player chooses the dialogue, but there are people who might choose other. The point of the game is that the player is the driver behind the wheel of companions' fate for the whole story, reducing MC input to being silent observer simply goes against the theme and game design. To make Wyll follow Karlach automatically when MC is not there u would probably need edits to the cinematic and probably new voice lines to simulate the second phase, considering Larian announced Origin characters' stories as finished its doubtful it will happen.
Regarding the engine there's no way of knowing this without metagame knowledge, besides looks like the they wanted to have good ending so that knowledge would be useless anyway, while playing the game for the first time as Astarion origin the only possible fix telegraphed by the game is the ascension, in tragedy setting you need to take every chances possible can't count on luck.
