When you claim as resident of a country, you have to report your world-wide income for that period. That means your wife has to report maternity EI to both usa and canada. However, you can claim foreign tax credit if there is any after you minus total tax paid by all refund from canada for your wife. I can confirm that you do not need to repay maternity EI, but have to report to canada when you leave and your UCCB and CCTB stop after your child were outside of canda and thus you have to return UCCB and CCTB the part you are not qualified.