Is a shul allowed to ask an on-call cop or E.R. worker, etc. to be a shat"z (shaliach tzibbur) if there is a fairly high likelihood that may be asked to leave on an emergency in the middle of davening?
Are there certain parts of the service where it may be permitted vs. others? For example, perhaps he shouldn't be shat"z for the Amidah (as that's considered more "important") but it's OK for Psukei D'Zimra).