Re: Timelines
If you read the blog post on the pi website you will see both a statement from Eben that
we use an Ethernet jack with the appropriate taps on its windings, and connect those taps to an additional 4-pin header, located just beneath the top-right mounting hole. A HAT can pick up these signals, request power from the switch, and regulate the resulting 37-57V DC down to 5V to power the Raspberry Pi.
The key word seems to be "request". There is also a table which claims the PoE+ HAT has "current sense". As I understand it (and I'm quite prepared to be wrong) a switch compliant with "at" will not attempt to deliver high power unless it is specifically requested by the client, thus the client must "negotiate", as you put it, if it wants high power.
I note your question about Mode A / B, but there is also an actively monitored comment section below the article where you can see that Eben and Liz directly respond to queries. Ask away.
Personally, I'm fuming that two bits of TP-Link wireless kit I've bought appear to have such a proprietary form of "passive" PoE that it would actually be dangerous to plug them into a normal PoE switch. The injectors supplied with the kit weren't a problem for a couple of years, but I've just reorganised everything and it would be great to tidy things up a bit...
M.