I can help with some of these:
1) If I run: ols-fwloader -f BOOT -n -V -w PIC_firmware/OLSv1-firmware-v2.1.hex
I suppose this will fetch the PIC firmware and compare it to the file named, this way I can check if I'm at a certain firmware level. There seems to be no way to determine directly at which firmware level I'm at.
In ROM update mode the program will read and report the bootloader, PIC firmware, and hardware version. There is no way to tell the FPGA version without reading the bitstream and verifying it against a known source.
4) I suppose I have to place the OLS into 'update mode' (ACT and PWR leds lit) before launching 'ols-fwloader'. I don't quite understand the '-n' 'enter bootloader first' option. Does this option allows me to run the 'ols-fwloader' without the OLS being placed in 'update mode' ?
No. The bootloader is entered by placing a jumper between PGC and PGD - OR - an application can send a command and jump to the bootloader from ROM update mode. -n sends the command and jumps to bootloader mode after updating the ROM.
5) Last, but not least, when I run the command 'ols-fwloader -f BOOT -V -w PIC_firmware/OLSv1-firmware-v2.1.hex'
I get the error 'Device not found' (independent of the OLS being in 'update mode' or not). I can access it using the Java sump client with no problem, though...
If you do not use the app to jump from ROM to bootloader mode, then it will not be detected. Try using the -n option, or enter bootloader mode manually by connecting the PGC and PGD pins of the ICSP header and resetting. The OLS will enumerate as a HID device.
I highly recommend Logic Sniffer 101, it has an introduction to the different modes, how to enter them, and what they do:http://dangerousprototypes.com/docs/Logic_Sniffer_101