They have the food stores, the self-reliance, and the building ability -- but the attitude towards contagious diseases, at least among the local-to-me Old Order Mennonites, is "God will protect his Church". Both accidental and disease-caused deaths and injuries are taken as being God's will. Vaccination rates are very low and masking was only practiced when legally required and often not then unless strongly enforced by authorities outside the church.
How they would react to the appearance of what appears to be in effect actual demons I don't know. But I very much doubt that any effective reaction would be fast enough -- unless, of course, in that universe their version of God actually spoke to them, before it was too late, to warn them and tell them what to do.
They'd definitely have the rash illness spread through the community. It might have a delay reaching them just because they're insular to outsiders, but it would get there. Maybe enough of a delay to know about trolls and prepare, except it raises the question of what would be believed.
It doesn't seem like following the professional advice made a difference in the rash illness because it hit so hard and heavy - and trusting vaccines screwed over the Danes in y0.
I believe they're good with guns for hunting and protecting livestock, and most farm equipment can be used to effectively kill, so between that and fences should be able to defend against trolls.
Trolls from the outside would likely be easy enough to address, but yes there would certainly be despair when their own people change.
I don't know if or how long it would take them to do mercy killings of people who get the rash illness. Or what their views are of suicide if people would do like Tuuri did.
It seems like a lot of the problems that scandinavia faced wasn't just losing people to the illness - but losing their resources and having to cobble everything together after fleeing the cities.
I don't think amish communes are typically populated enough to need to flee, and once the initial waves were survived they'd be able to get back on their feet faster than their modern contemporaries.