There's no such thing as 'secondary stadiums' for the World Cup. Instead, you have two stadiums from the one city.
When i say secondary stadium I'm just referring to the second venue selected from Melbourne or Sydney; not a second-rate stadium.
They'll arrange the matches so that there won't be any days where a city doesn't have two matches on the same day, then the MRS/SFS/Docklands would only be given four or five group stage matches but no round of 16 or quarter finals.
I'm not quite sure what your trying to say here...
They would never schedule a city to host 2 matches in 1 day.
Luck could have it that Argentina and Croatia are drawn to play at the so-called 'secondary stadium'. And there's no chance in hell that FIFA are going to alter the venue for them.
The draw will be configured so that all the matches involving seeded teams will be treated as "blockbuster" matches and drawn to play at the larger venues (i.e MCG, ANZ, Suncorp, New Adelaide Stadium etc.). SFS or MRS would only host matches involving the remaining 3 teams from a pool; hence Argentina v Croatia would never occur at either of these venues.