The vent is crucial for a sewage ejector. Some codes require that it be vented separately in order to insure that it does have venting that is not compromised by other fixtures which might be in use.
Sounds gross, but the sump does not empty completely when the pump runs. Thus some "bioeffluvia" remains and will be a source of "pungency". Sealing the unit within the basement and having an adequate external vent are necessities.

Install the backflow device first. If the problem presists after installing backflow,try sealing up and installing a vent valve on basin that seals off until basin drops below atmospheric pressure. You need the backflow anyhow and the one-direction vent is inexpensive and easy to install.