My account is also SG based. I’ve often upgraded a T/T to Sydney using Avios and there’s always an additional charge for WTP to CW but shouldn’t be for CW to F - as one of the other members mentioned, this is due to different BA surcharges. Sometimes it is a bit cheaper to do by phone (gold line for me as I’m not GGL) but at the moment it seems the same regardless. Eg. The T/T fare in the current SG sale is about 1400 SGD, but you’ll find that rises by about 150 SGD if you upgrade both legs to Club World - or a bit more than half that amount if you only upgrade only the outbound.
I’m also always charged HKD when phoning the gold line but it’s usually a small amount so the FX aspect doesn’t bother me.
As to your our first question, I’ve noticed similar issues with London gold line when booking complex itineraries due to the impact of a Singapore base and more recently when trying to redeem a GUF2 voucher for a simple itinerary. Ie. Different fare inventory available to what shows on some search engines. So far I’ve found using Propeller Travel the best route for GUFs - particularly since they don’t need award inventory in the target class.