Updated : 04/08/2014
The 5-year period is counted from the date when you actually took up residence in Italy, not from when you first received a registration certificate.
NO - The continuity of your residence in Germany is not affected by temporary absences not exceeding a total of 6 months per year.
YES - To have the right of permanent residence in the UK, you must have been working, studying or supporting yourself independently in the UK, for a continuous period of five years.
Living in a country for more than 3 months is often described as being a "permanent resident" there. However, this does not mean that you automatically acquire, under EU rules, the right of permanent residence. This will be granted after you've lived in Italy for five years.
The certificate of permanent residence should be valid for no less than 10 years. You must apply for it or for its renewal, before your previous certificate expires. You could be fined for failing to do so, if national rules so provide.