A certain amount of oil will get past as when not on boost there is a vacuum in the inlet then, a step gap seal as said sorts about 90% of this.
I don't know about the union size hole, but the pressure will be the same so shouldn't make any difference.
The negative pressure thing you mention is interesting, some cars in the us ran with evacuusump, where they had the sump connected to the exhaust, in between each positive pulse in the exhaust there is a vacuum pulse so exploited this, with anti backfire valves in the exhaust so a back fire didn't blow the sump off!!
tabetha