When you use /EXTENDED=1024, you are asking the driver to leave 1MB of memory available as extended memory, e.g. for HIMEM.SYS to make available as XMS. In a 2MB system, that will leave 1MB behind, 640kB of which will be used for base memory, leaving 384kB available to the driver. This will result in up to 384kB of EMS. If you have shadow RAM enabled, that will come from this 384kB as well, leaving less available for the driver to make accessible as EMS.
On a 4MB system with /EXTENDED=3072 then you should have exactly the same result - only a maximum of 384kB available as EMS. I am not sure why this didn't work for you, I have tried a similar setup (but /EXTENDED=2048 as I wanted 1MB of EMS) and it worked fine for me. I would suggest using the /FULLDISPLAY option so the driver prints out how much EMS is available and what the page frame is, during boot. It might give you some more information to explain what is going on.
HT12MM.SYS does not read the page frame address from the BIOS (not all BIOSes have this option - mine doesn't), and picks the first address starting at segment C800. On my 286 I have the XTIDE BIOS sitting here, so my EMS page frame ends up at segment CC00 instead. You can use the /FRAME option to manually set the page frame, although it doesn't really matter where it is. You'd only ever want to move it if it (for example) sat in the middle of two UMBs, and by moving it you could turn those two small UMBs into one larger UMB. But since the 286 boards in question don't provide native UMBs, moving the page frame is not very useful.
You don't *have* to have two steps to get UMBs on a 286. You can load the EMS driver to get a page frame - a page frame is basically a UMB itself where the contents change in response to EMS commands - and then load a second driver that essentially blocks all EMS commands, and tells the system there is a UMB available where the page frame is. This looks like what you've done.
However in theory you can load a UMB driver that has proper chipset support, and this will just program the chipset to provide memory in the UMB area, and then tell the system that those UMB blocks are available. While there doesn't appear to be a driver that does this with the HT12 chipset, in theory it's better because the EMS page frame can be from 16kB to 64kB, but if you program the chipset directly I believe some of them can go up to 128kB. You might also have a chipset one day that can mark shadow RAM as read/write, which would give you full (up to 224kB with EGA/VGA) UMBs without needing an EMS driver. I realise this doesn't help you, I just wanted to be clear that it's not 286es that require the two step process, it's the chipset you're using and the lack of driver support for it.
I believe XMS-UMB is not an option on a 286, as this works by relocating extended memory back into the UMB area - something which the 286 cannot do unless paired with a chipset that specifically supports it. This is perhaps why this function is missing from HIMEM.SYS - they have EMM386 to do that, and it of course only works on a 386+.