We too will be away on May 6th. We shall be driving from Gairloch to Rothesay as we will be on our holidays.
Forewarned is forearmed is my maxim, so I went to the official site click here where I was able to fill in my details on a postal application form and then print it. It even gave me the address to send it to, the whole process was so simple even I could do it :o))
Not as far as I'm aware - it would be fraught with security issues. There have been trials with electronic voting in the UK, but there were serious concerns about the potential for vote-rigging, and I don't think those are near to being resolved at the moment.