The problem with an inland ap and leaving the country during processing is that it is required that the applicant be resident in Canada and residing with their sponsor/partner in order to be approved - basically because the landing interview happens within Canada. If, after leaving Canada, the applicant is for some reason denied re-entry, the inland application fails because the applicant can no longer meet the requirement of being resident with their sponsor in Canada.
Even visa-exempt nationals have to submit to examination at a Canadian port of entry. It is entirely up to the discretion of the officer whether or not to allow entry. Allowing entry is supposed to be based on whether or not the foreign national can demonstrate that they do not intend to stay permanently in Canada . . . that they will leave at the end of their authorized temporary stay. This can be difficult to do if you have filed a permanent resident application and, in fact, foreign nationals from non-visa-exempt countries are routinely refused visitor visas when they have applied for permanent status. They cannot convince the assessing officer that they intend to leave Canada at the end of their authorized stay. So the officer at a Canadian port of entry has the "right" to refuse entry, even to a visa-exempt national, for the same reason. Whether they will or not is the risk you take if you leave. Sponsorship approval will not guarantee re-entry . . . and, although having an open work permit might make it a bit easier, you're still putting yourself at the mercy of someone who is not obligated to do anything on your behalf.
