We installed a Lavac in IDUNA. My observations regarding the vented loop in the inlet hose follows:
The size of the hole or number of holes in the supplied "vent button" determines the amount of water left in the bowl. Lavac, England, recommends starting with a small pin hole then increasing the number of pin holes to control the amount of water left in the bowl.
1. The smaller the hole, the more water left in the bowl and the time required to break the vacuum in the bowl increases,
2. Increasing the number of pin holes in the vent button, decreases both the amount of water left in the bow and the vacuum break time.
The discharge hose loop only needs to be above the waterline for overboard discharge.
I see no reason that would prevent discharging the head directly into the holding tank without an anti-siphon device installed in the discharge line. Our Lavac manual is on the boat but I believe Lavac shows this arrangement. We will check the manual this afternoon.
To decrease the amount of water pumped into the holding tank, we pump the "vacuum pump" about three times, close the inlet seacock, then pump the head until liquid ceases to flow through the holes. The "flow gauge" is an estimate based on the input feel from the pump handle and the amount of air sucked into the inlet hose vent button. This is determined by the sound the air makes as it passed through the pin holes in the vent button. You probably already know the sound.
Our general observations about holding tanks is a 20 gallon tank will last perhaps 5 days for two people with careful use of the head. This subject is probably one of common topics between cruising boat owners. After the city of Baltimore dumped several thousand gallons of raw sewage into Baltimore's Inner Harbor last year and turn the harbor water green with algae, it sometimes makes one wonder how much holding tanks help the Chesapeake. It was officially called an algae bloom but the cause was never told to the public - "backdoor politics."
Lenora and I viewed the images you posted. Whitewings II is beautiful. The contrast between the dark hull and the deck/cabin sides are striking. She appears to be well kept and is a testimony to her owners care.