I'm trying to do some forward planning for a trip to Thailand next year, when I expect to spend some time staying with family.

Just where can the TM.30 reporting be done? The instructions say, "If there is no immigration office in the province or locality of the respective house or hotel, the notification is made to the local police station", but I don't know what is meant by 'locality'. For example, if there is no immigration office in the amphoe, can one use the local police station instead?

If not, if alien and householder are travelling together, can the householder pop into the immigration office en route?

If the householder is absent (e.g. for work or because of sickness), can his spouse do the reporting? If not, what happens?

Finally, do Thais that enter the country on a foreign passport have to be reported?

I don't think any police station would accept a TM30. It would have to be done at the designated immigration office for where you are staying.

In reality you probably will not need to do a TM30 unless you have a need to do something at an immigration office and not all offices will ask for it then.

It a Thai enters the country on a non Thai passport the same rules apply for them as for any foreigner.

TM 30 can be sent by registered mail.

TM 30 can be sent by registered mail.

There is no allowance for that in the immigration act. Some immigration offices may accept them sent by mail though.

Immigration Division 1 (Bangkok) allows this as per web site and personal experience.