I feel that Sunday works well for the HFLW. A number of those players still play women's sports such as netball, and the Sunday slot allows them to play both. HFLW is working VERY well in the hills at the moment and is growing in participation, with a lot of spectators watching on a Sunday arvo with a beer or two. Putting the Women into the Saturday slot would also leave Girls football (which currently is mostly played Sunday as well) hanging out on a limb.
Building on comments you have put forward, it seems to me that Country Division sides (who always have the two senior teams - just missing juniors basically) could host a lot of the modified junior football that is played on Friday nights, rather than your suggestion of HFLW. No doubt there would be programming challenges with that though. This may be an alternative to the idea of trying to have all players in one comp. Certainly, allowing a tier 2 level comp to exist saves repeating the error of 1983, when there were many extremely one sided games.

It would seem obvious to have Mt Barker Football Club be the primary tenant. Perhaps the HFL with the head lease (for football months) subletting it to Mt Barker Football Club for minor round games. Maybe then, finals played there under HFL banner - but the old problem is who runs the services like canteen, bar and BBQ areas and who keeps the money. As we all know, if you have to pay people to do this stuff, there is much less revenue. Clubs hosting finals is also a much needed revenue source for clubs, so the existence of this facility may change that dynamic as well. Also, is the area fenced off to allow gate sales on any match day? Big miss for a solution if as I suspect it is all open.