1) up to you, my C70 has 120K on it and was running fine, Volvo informed during a service that the free ETM clean & update had not been done, as it was free and they had the car I said do it.
2) that information is contained within this ETM section, I'll try to find a direct link to the technical aspect (but IIRC they clean the tracks on the throttle module).
3)
http://www.vexedvolvo.com
4) yes
5) no, everything like performance, economy stays the same.
6) yes
7) No, it something tha haunts all the 70 series models, some are more unlucky than others ...
8) No, its more preventative measure but does not eliminate the threat of the ETM failure.