Oh Rosy, after reading that, I miss David as well!
First, it was massively brave to go away that first time on your own - and now you have done it and survived so you will know better what to expect next time.
Second, if he is a prospective neighbour then I think the onus is on him to prove the boundary is wrong, not you to prove it is right. I hope your BIL told him he needs to instruct a solicitor - you are not under any obligation to provide him with information and it is the seller that he should be asking.
Third, the lost emails - if the worst comes to the worst, you could simply spread the word that all is lost and ask your friends to resend from their outboxes. No-one will think that is weird.
I can't offer you any comfort about the M25 - motorways and business travel are the bane of my life and I think 'wobbly' is a perfectly reasonable reaction.
Finally, I know there is no point saying I hope you had a happy birthday as of course, you didn't. But I do hope that it was as nice a day as possible and filled with memories of happier birthdays in the past.
Keep going lovely - it might not feel like it but you are doing great x