Does concealing the obligation for non-scriptural spouses to separate after one of them converts to Islam constitute deception in Da'wah (proselytization)?

There is no harm in omitting the mention of some legal rulings to the one being invited (to Islam), as this is not deception. Rather, it is part of the wisdom in Da'wah (inviting to Islam) to proceed gradually, observe priorities, and begin with what is most important, then the next most important, just as the Prophet, peace and blessings be upon him, did with Mu'adh ibn Jabal when he sent him to Yemen.