Upvote:4
We have many sad reports at this site from people who stayed longer on their visa than the original itinerary used to obtain it, whose subsequent applications were denied. However, these generally involved much, much longer deviations than five days, that suggested either the purpose of the visit (tourism) or the economic circumstances (how did you live for months without working off the books?) were not stated correctly in the application. I would not recommend staying until the last possible day, especially since a flight cancellation will make many more problems for you. Others may differ, and will downvote, but I think one or two days would be OK, if you feel you need them.
If you enter and exit at the places mentioned in your application, no one will care if you are coming direct from your Home Country or via a third country. It is understood this would sometimes be the case for cheaper flights even without additional holiday at a stopover.