Are there any benefits when booking VA as a SQ KrisFlyer Gold status member?

Conditions seem very murky and hard to decipher. From what I gather, as long as I can book the flights as SQ codeshare flights executed by VA, I should be getting points and lounge access. But what about when I fly on VA ticketed and flown legs on Oz? Do I get lounge access as a KF Gold member?

Yes, KrisFlyer Gold and PPS members get lounge access in Australia prior to all VA flights, whether booked on a VA or SQ flight number, including on domestic-only itineraries. You can also use Premium Entry in Sydney and Brisbane, and get priority check-in and boarding, etc.

Awesome, thanks!

I'm assuming status includes the privilege to preselect one's seats? How does one go about that when not a VA member but Kris/Gold member?

If your number is attached to the booking, the seats you can see are the ones you can select. That won't include Economy X, though, but I'm not sure that VA gives any preferential seating to SQ elites.

I know that when I've flown with VA and my Alitalia Freccia Alata (Gold), Etihad Gold and Airberlin Gold numbers have been attached to bookings, the seating options were the same as open to Velocity Red members.

(Yeah, that Airberlin Gold card isn't much use any more...)

Just remember to bring your KrisFlyer card with you to present it at reception if necessary - and if you don't have the physical card, you can usually show it on your smartphone (such as via Apple Wallet if you've saved the card there).