A valiant effort
However, I'm afraid it was a trick question. Not intended to be cruel, but to help make a really important point:
Only ONE device may be connected to an isolating transformer at once.
This has nothing to do with power ratings, which is the usual thing that springs to mind. Rather, it is related to safety.
The devices might well be isolated with respect to mains earth, but as soon as you connect a piece of earthed test equipment to one of them, then that device is no longer floating. But,
all the other devices stop floating at the same time. And depending on the exact circumstances, this could create all sorts of unexpected hazards.
The worst way this could manifest itself was in the old TV workshops that had a massive isolation transformer that powered many outlets simultaneously, with several engineers working on different sets all at once. All was fine until Engineer #1 needed to hook up a 'scope to observe a waveform on a set - this means that all the other sockets are now referenced to mains earth in some way. If you were unlucky enough to touch Live or Neutral, then you'd be in with a chance of getting a shock - that, of course, is the risk that many people believe the isolation transformer is meant to eliminate (never make that assumption!).
Moreover, suppose Engineer #2 needs to connect his 'scope to the chassis of the set in front of him. This could be lethal - if set #1 uses a full-wave rectifier and set #2 uses a half-wave arrangement, then the fault current flowing through the earth path (via the ground clips of the 'scope probes) could be massive - assuming you manage to make that connection without getting a serious shock in the process.
It's a miracle that more people didn't die back then. It's also amazing just how much misunderstanding existed back then, and persists today.
A document that I frequently refer to is attached to this post:
https://www.vintage-radio.net/forum/...8&postcount=18 - it covers isolation transformers in workshop scenarios in decent detail. Well worth a read if you haven't already seen it
Thinking about your workings, I'm no expert on transformer ratings, but I believe that a 500VA transformer should be able to provide 500VA at the output, so the "waste" energy should be added to the primary rather than subtracted from the throughput - so 575VA in, 500VA out. In practice, the losses will rise because of I-squared-R losses in the windings, amongst other things. And if the load is anything other than a resistive load, there will be de-rating to consider. Luckily, as the failure mode is thermal, they are fairly tolerant to short periods of abuse.
Hope this helps,
Mark