You can do a TM30 report online but a first 90 day report (TM47) usually has to be done in person. A 180 day stamp makes no difference, you could do two 90 day reports on that (if you do the first report early).
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.