What is the most optimal manner (least amount of replaying) to do all faction quests? I'm trying to get all 4 achievements for completing the respective factions' questlines.

My progress in the game is as follows:

Main quest: started "He Waits in Fire", but yet to sail to Ashen Maw

Huana: started Fruitful alliance

Principi: started Shrewd Proposition

VTC: Of Like Minds

RDC: Clearing Out Crookspur

Should I, before even landing at Crookspur, complete He Waits in Fire?

Thanks.

If you want to get all achievements in one PT you'll have to reload an older save before the lock-in.

Some faction quests are available at any time and some are available after a certain part in the game. For instance in order to do the Huana one you'll need to have completed Hasongo and received a message from the Queen. You can actually complete the Crookspur quest before that one by going to Ori o Koīki first and get the quest there. You'll still need to talk to the queen to progress further though.

The final lock in starts after He Waits in Fire and you have to choose a faction to join. Once you do so all other faction related quests will fail.

Is the lock in automatically forced on the player as soon as HWIF is completed, and one has to choose there and then?

No, the queen will invite you for a meeting with all factions. After this you have to choose to go to one of the factions and talk to their leader. Agreeing to help them will start their final quest and will cancel the others.

Got it, thanks.