Yeah, so if the Brewers win tomorrow AND the Giants Black lose...the Brewers will advance based on head-to-head. As you point to, however, if the Giants Black win and the Brewers Gold win they revert to standard MiLB post-season Group Play tiebreaker rules. Now, I'm certainly not an expert in this area but I think MiLB does not use the standard run differential as tiebreaker criteria in the post-season. I think what they would do, again I'm not sure this is entirely accurate (anyone who knows for sure please chime in) but this is how I understand it.
Some sort of this type of assessment would be used:
(1) In this case, we would have three clubs. SO it wouldn't be a head-to-head best record it would be best win percentage in games versus the two opposing clubs amongst the three teams
(2) IF there was a tie stop that scheme? Which of the tied teams had the best winning percentage over the final 20 Games of the season?
(3) IF still tied (improbably so) over 21 Games? 22 Games? etc. until a team came out on top.
As convoluted as that is (LOL), I do think that is how they'd go about it.