I would not worry. If the house potable water supply dropped to less than the boiler head pressure, You would certainly notice the lack of water flow and remedy that situation long before any boiler water entered your potable system. But you do have a check valve on the boiler feed meaning that boiler water can't enter the potable system. If you like, check the check valve. Turn off the main supply and drain the potable water system and watch the pressure gauge on the boiler. If it doesn't move, you know the check valve is working.
If you have city water you should never have domestic water pressure problems. If you have well water then only when the pump fails or you pump the well dry will you have water pressure issues.
If you have further concerns, please ask. And you can always duplicate a check valve if you like.