I've gone back to parallel plumbing too. I swapped to series plumbing previously, and due to where my vapouriser is, either way I have pipework across the back of the engine to it - but figured that it would be better.
To make it neat, I used a solid metal 90 degree elbow, as the hose run would have just kinked it to get it around the back of the engine neatly. The result - a luke-warm cabin as there wasn't enough flow through the heater matrix.
Mine was plumbed in parallel when it was installed, back in 2008, and I never had an issue with it cooling off the vap. I finished off the LPG plumbing today so I can get it set up to run on gas again with the new engine, and the reducer was still warm when I touched it, about an hour after getting home - so I really don't think it's going to freeze over in my case.
I've done my plumbing so that the hoses run around the back of the engine and are lower than coolant tank. Also as most reducers are 16mm, and the RR hoses are 19mm if you use the 19-16-19 T pieces available, then you don't have any extra pipe reducers either.
I now have a nice hot cabin again :) (sorry Morat - not rubbing it in! - we'll have yours sorted soon!
'06 4.2 Supercharged Black/Cream "Jess" -- Bought April 2021 (NZ)
'01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
'99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
'95 4.6 HSE Epsom Green/Tan -- Traded June 2008