I do not live in Scotland so do not track closely the difference in the rules between the two parts of the UK - I know a reasonable amount about England.
However this document may help you.
http://www.scottish.p...Finalconsultation.pdf
Written by John Wilson, MSP, it confirms that the statutory limit for clawing back council tax in Scotland is 20 years. Maybe John Wilson's office can help you since it seems to be a topic that he is keen on?
I suspect the issue is also going to depend on whether the arrears have build up because the local authority made a mistake in assessing annual figure - or whether you, perhaps inadvertently, give incorrect information or incomplete information that prevented the proper assessment from being done.
The answer to the second part of your question is that the SPSO is the ombudnsman for dispoutes with local authorities in Scotland - once you have exhausted the complants procedure locally with the council. See here: -
http://www.spso.org.uk/