Water always seeks its own level
So yes, if the tank or any part of it is below the waterline, a vented loop is needed, because water will rise in the tank to the waterline through any open thru-hull...the macerator may slow it down, but it won't stop it. The loop goes between the macerator and the thru-hull and should be 8-12" above the waterline at any angle of heel. However, a vented loop will not prevent water from being forced up and over it by the pressure of the hull against the water while underway...the seacock must always be kept closed except when actually dumping the tank. But the whole point may become moot, because Pilot Bay on Kootenay Lake has already been designated a "no discharge" zone (see the list of BC "no discharge" waters at http://www.pacific.ccg-gcc.gc.ca/obs-bsn/rias/index_e.htm) --which means it's illegal to discharge a tank directly overboard OR dump a tank there, and other parts of the lake are likely to follow. As for the head intake, the same principle applies: if the toilet is at or below the waterline--which it is on most sailboats--a vented loop is needed in the head intake to prevent water from rising in the bowl if the wet/dry valve is left in the wet position. The loop must NOT go in the line between the thru-hull and the pump because it will interfere with the pump's ability to prime...it must go between the pump and the bowl, and should also be at least 8-12" above the waterline at any angle of heel. Both seacocks should always be kept closed when no one is aboard, and should really remain closed at all times except when actually in use. However, because boat builders have taken to putting thru-hulls in locations that are so inaccessible as to make that all but impossible, keeping the head intake seacock closed except when the toilet is in use may not be practical. So since a vented loop won't prevent water from flooding the bowl while underway on a tack that can force water up an open thru-hull, it's advisable to install a shutoff valve in the head intake line ahead of the pump that is accessible and can be kept closed at all times except when the head is in use. I suggest you check out the link below. Several chapters provide detailed instructions for installing a toilet and holding tank correctly.
