Yes, a very silly installation as you say. The macerator connections do sound completely normal. I still suspect a fault somewhere in the float switch.
In fact, installation instructions do insist that there should be an access panel, one that is larger than the unit so that it can be removed for maintenance.
If you plan to keep the unit, I'd strongly advise creating access via the plasterboard. A very simple operation usually. I'm sure you're familiar with the sort of thing I'm talking about. Until the unit can be got at, we're just guessing.
If only "water", then certainly a 38mm run is all that's needed. You know this would be against regulations though............. but sometimes, one has to be "creative".
After all... a Gents urinal needs only a 38mm diameter waste pipe. At the risk of being indelicate, there is a female version I think (don't quote me though. I'd need to do some research.)
The main problem with fitting a 38mm run to a WC would be when it's flushed. The volume of water could easily overload the pipework's capacity to remove it........... then the pan overflows.
Altering the amount of water delivered per flush could overcome that though. (Lower the float level in the WC cistern... and reduce the "travel" on the operating cable/lever to deliver less water per flush.)
I don't see why that wouldn't work ............ just don't let any guests use your loo though.