ok, sorry I can't provide the link (but I think the RTE London wensite FAQs might say that 6 months validity is not required but better to have an Immigration link).
You are correct but many airlines have the 6 month rule regardless of the final destination. I think your friend needs to contact them when you get the Thailand entry proof to see their response.
it doesn't "have" to be 10 months to meet the requirements, two months before applying then 3 months after does it need to be 800k then 2 months before your next extension (having never dropped below 400k) to use the 800k method for the subsequent year(s)