I wil always say DnECUDiag (lame name, I know) is better than MultiECUscan.
About your problems:
1) Use the software to find out the errors that are popping up.
Apart from the (free) software, you need a cable. You can get them on eBay for 10pounds, or a guy here (rarebear, I think) sells them as well. His can be used for Airbag and ABS resets as well, I think. I would say that having a software for yourself is a necessity for Selespeed cars. Too many "mechanics" out there don't know how to work on these.
To use it, download the software, use the cable to plug in the laptop to the car, set the key in car to MAR, launch the software, select the Selespeed ECU in the software, and in the fault codes area, you should be able to get readings. First stop for any fault finding.
2) The downshift paddle that does not work; does your lever work on downshift ?
If it does, its probably a loose electrical connection. You can test with the software, or the old fashioned way, by checking the wires.
3) The crunch is probably the gear synchro, as said. Its purely mechanical..
Good luck, and welcome, hopefully you will get these problems sorted and it will all be alright!