The waste pipe from the head (outlet) goes directly to the sea cock without a vented loop in - is this normal?
The inlet ditto, no loop in it.
That's how it is on our boat, no problems with it. Both hoses are looped (not vented, just looped) upwards well above the waterline before coming back down, but no siphon breaks in them.
We have a holding tank on our 40 Ocean, behind the (removable) shower wall, in the curvature of the topsides. There's a Whale 3-way valve in the toilet outlet, which lets us choose whether the toilet output goes straight to sea or into the top of the holding tank. The holding tank is gravity drained, with a separate outlet seacock. This system works very well, we never had any problems with it (after initial cleanup of massive calcium deposits from previous owners).
I've just replaced the diverter valve (after 18 years it started to leak - acceptable service life). Was pleased to see that with our strict 30 pumps regime (calculated from hose length), we had zero calcium deposits in any of the three hoses connecting to the valve.
When we open the tank seacock to empty the tank, we always flush a large amount of water through, and looking in our wake that seems to empty the solids which don't always come out without the extra flushing. Doing this, it always empties completely.
Toilet paper obviously never goes into the toilet, but into a separate bin bag (with airtight lid).
This setup and usage has been working flawlessly for us for two years now. The only issue we had was prior to the 30 pump flush regime from calcium deposits, never any since, and some normal wear maintenance on the pump and now the diverter valve. Avoid toilet dramas ;-)