You'll have to remind me if this is Bosch or Gems, I probably don't fully remember intake ducting arrangement for Gems but all air including IAV passes through the MAF on Bosch(?) and I seem to remember (though now expect to be corrected) it does on Gems? Gas entering downstream from the MAF is still gas, doesn't detract from the air the engine needs.
On quite a few P38 LPG conversions, and in fact on quite a lot of other vehicles too, I have seen engine load decrease at idle running on LPG compared to petrol. Some of this can I'm sure often be attributed to new LPG injectors supplying more equal cylinder to cylinder mixture than old/worn petrol injectors but I think there's a bit more in it than that because the same happens on some engines even with petrol injectors known to flow exactly the same - I believe it's related to the difference in stochiometric ratio. Another advantage (in some cases) of a mixer over injection is that if there's any difference in airflow cylinder to cylinder (which can effect cylinder to cylinder mixture in the same way as worn injector flow rates) a mixer will see that each cylinder gets the same/correct mixture anyway.