Hi, sorry for the delay...
> 1. would it work with a Maple RET6 (it has the STM32RET6 chip - 512KB Flash) (which .bin file to use in that case)?
I wasn't aware there was one... Is it out of beta now? You could try the standard Maple binary and see if it works (it just might), or the HY board uses a 103VE (but the pin names will be wrong and it will be expecting an LCD!) - otherwise I'd have to get a board and re-compile it. To be honest the list of supported boards is currently greater than the amount of people that have registered, so I'd have trouble justifying it right now!
> 2. how much memory does the interpreter take up in flash?
Around 100kb - on boards with >128kb flash it's around 160kb as I compile with more optimisations.
> 3. if you "save" a program to Flash, is the a way to see how much Flash memory is left?
Espruino pretty much saves the whole memory area in to flash (right at the end of the flash), so if you have 20kb of RAM on-chip, it's using almost that much flash. There's currently no way of writing the extra flash memory from Espruino either (but you can use peek32 to read :)
If you need to store lots of data you could always use an SPI flash chip.
> 4. are you aware of any other editors that could be used with this instead of typing everything into a terminal window and trying to edit the session in the terminal window? I.e. is there an editor where you can just type in your whole JS program and then send it over serial to the board, make changes/repeat?
On Linux you can open the serial port to write while it is already open by a terminal app, so you can use your favourite editor, and can then create a custom tool command: "echo 'echo(0)' /dev/ttyUSB0;cp myfile.js /dev/ttyUSB0;echo 'echo(1)' /dev/ttyUSB0"
Note the echos - which stop Espruino spamming the console with everything in the file. As far as I know Windows isn't quite as friendly though and I'm not aware of a program that does it...
> 5. do you need to buy a separate registration code for each of the boards one owns?
Yes. Only if you want to save to flash though - if you're happy to program the board each time you power on (eg. if you're just using it as an IO board) then it's totally free.
If it gets popular enough I'll add a lower cost option for subsequent registrations, but to be honest it's pretty cheap already. An STM32 Discovery + Espruino is about half the price of an Arduino Due - it's just that in Espruino's case you actually notice paying the money that helps fund the software development + documentation work :)