I'm not sure if this question relates to Buddhism or Thai tradition.

Why are funerals in Thailand always scheduled for the afternoons with the cremation at 3pm?

3 to 5 pm - don't know why.

what was strange during the funeral I attended, that official from the muang gave a speach and not member of the family

Not every funeral in Thailand, I've been to literally hundreds (DH has a lot of relatives) and the cremation takes place anytime in the afternoon, there is no set time. The Wat in our neighborhood usually cremates around 2pm but I suspect it depends not only on the family but on the abbot as well.

Around here the funerals start about 11:00. The timing can also depend on the number of funerals. On the day of Dale Henry's funeral in Ranong, his was one of 4 scheduked for the same day.