The best I can come up with so far, given that canon indicates that people come to the monastery for training before returning home to defend Sommerlund is something along the following lines:
Many suspected of having hereditary ability are sent by their families to the monastery (either after a Kai Lord has tested them for innate ability, or for actual testing). In addition, the order seeks out those with ability.
Of those sent to the monastery, some leave the ranks of the order and return home once they are "trained enough", while others "feel the calling" and stay on. The rules allow for ex-Kai, so I feel there should be some, somewhere
Every current member of the order, without fail, returns for the feast of Fehmarn.
This does seem a little impractical, however, considering that there must surely have been some undercover operatives working during the feast... still, in lieu of a canon explaination I'm happy to assume that they all felt it important enough to drop what they are doing (in some cases on the other side of the world) and returned for the feast, at least on that one occasion
There are two exceptions I'd allow for "survivors" of the massacre.
1) Those who have peacefully renounced any ties to the order (see my final paragraph for details of why they wouldn't have helped rebuild). Examples would be those who wanted to return home after training, those who failed at some stage of training and were asked to leave, etc.
2) Vowbreakers. As is stated in the rulebook, these are hunted down ruthlessly, so they're either dead or happy the order were massacred (or possibly even helped out...)
Of course this raises additional questions about just how far you can go before you are branded a vowbreaker...
A final issue is worth mentioning. Again quoting from the rulebook, those who turn away from the teachings of the monastery are never allowed to return. I assume LW thought this principle important enough not to search for any ex-Kai and ask them for help in rebuilding, or in some cases they may simply have had any interest in helping.