Jan,
Perhaps instead of beginning from a state of your own inventive logic you should consult your own version of the LAA.
Certainly in England long before we volunteered to move away to float her further off the Continental shores, the LAA following (I suppose) Rotax' advice have demanded a small 35 jet bleed off the main post pump fuel pressure piping. And to return it to a handy fuel tank & not back to the pump inlet, or recirculate to the inline fuel filter/gascolator either.
As folk above ave explained it does have a minor function and removes air bubbles or vapour whilst having no effect of the carb's feed pressure. This bleed back also allows pressurised fuel, when static with engine off & hot to escape back to the tank and this helps stop carb float chambers perhaps temporarily overflowing with excess pressure and the consequent mess plus fire hazard.
In my experience the return quantity with the original PIerburg pump during flight is in the order of 2 to 3 litres per hour.
In my a/c a Rans S6-116 which is fitted with the current bog standard overhead 2 X 35 litre wing tanks it's common sense when both tanks are full to run off the fuel returned tank till it's gone down enough to accept the bleed feed 'topping it up'. Easy to see by the pilot either from the simple clear pipe gauge or easier through the translucent tank wall on view.
Lastly those "annoying" pipes from the carb's ! They have a serious duty.
You really ought to re-read the Rotax free on-line manuals which explain quite lucidly their purpose. They are to monitor the ambient pressure and deliver that to the float chamber. Pressure above or below this do affect the 'push' on the fuel up & through the carb jets to either enrich or weaken the mixture.
That's how some of the common 'leaning' devices work , because they insert a bleed valve - screw adjustable - suction source to these pipes and by the P1 juggling the screw against EGT's he may save a little petrol/gasoline/benzine per hour.
Most of us are happy to go with whatever Mr Bing delivers when set up to Mr Rotax' spec's.
In fact I'm surprised your Inspection authority didn't ensure the return bleed from pump outlet to a tank wasn't done when the a/c was first assembled. It's not difficult - a standard Tee piece is put in with three 13 mm pipe clips with a 35 jet on the return leg and a pipe following the same route back to the tank as the regular feed. To get it into the tank is another question, but either a Tee in the sight gauge pipe (like mine) or at a guess in via a tee into the tanks top breather/overflow right as it goes in at the neck ?
Better solution is please ask your Inspector.
Regards,
mike hallam