You can’t have it all. The least out of pocket option is one ticket with SM, and the other two with credits. The downside is the SM purchased one will be a different PNR and won’t be inside your app. However you can log in as that other person.

The only way to have as one PNR/all within your app is to purchase all three at once but you’ll have roughly $2k out of pocket for that convenience.

item 4 is incorrect. The perks are either from being on the same PNR or can be manually overridden by an agent for thing such as baggage if they’re on the same routing. Since you want to fly PS, what perk do you think is unique to status that you don’t also get with purchasing PS?

item 3 doesn’t help get either goal of least out of pocket or having all three on the same PNR.

advice: wife on SM, you and kid on credits. That way the kid doesn’t appear as an unaccompanied minor and require manual override at the airport to check in, etc.