Learn English – Unforecasted versus unexpected

...and try not to be shocked or overreact if...

In this sentence, you're telling them that what they must try to avoid is any of being shocked or overreacting. In essence, you're asking them:

...[try not] to [either be shocked or overreact]...

And not, for instance:

...either [try not to be shocked] or [overreact]...

Phrases with neither...nor are more explicit, but also more formal, and I think not...or is much more common for lists where all items are negative.

Stan Rogers has got the right answer but the wrong reasoning.

practitioners' is correct because what's being refered to is their community, so there's a possessive 's to be added. But because the subject is already plural, the actual s doesn't get written. Nor, incidentally, does it get pronounced in this situation, but pronunciation of the possessive s is independent of whether it's written or not.

practitioner's applies with a single practitioner for anything of his, including his community

practitioners is invalid with no possessive apostrophe, because all you're left with is a plural noun. That can't be used as a modifier to anything, including communities. You can have a dog trainer, where the 'noun' dog modifies trainer. But you can't have a dogs trainer.