VA Booking on SQ - Seat Selection & Premium Economy

[PP075]

With the recent changes to earning Status Credits on VA‑marketed flights — including the 50% cap on partner airlines — it looks like I now need to book my Singapore Airlines flights to Asia through Virgin Australia to retain Platinum.
When doing that, I can only choose Flex or Business fares (when available) on the VA website.
I’m trying to understand what’s actually possible once the booking is made. If anyone has booked a VA‑marketed, SQ‑operated Flex fare and then used the SQ booking reference on Singapore Airlines website:

1. Were you able to select Economy seats with extra legroom on Singapore Airlines and pay for them directly with a credit card?
2. Were you able to upgrade to Premium Economy through Singapore Airlines, select a seat, and pay the fare difference with a credit card?

I’d love to hear from anyone who has tried either option. Is it possible to pay for extra‑legroom seating or to upgrade to Premium Economy on an SQ‑operated flight when the original ticket was booked as a VA Flex fare?