First thing first, have a great holiday
Q1. & 4.
Your filter should be OK for two weeks without a water change, to aid this I would only feed the fish every other day because 1. reduces the risk of overfeeding and 2. will encourage the fish to look for any uneaten food that may have been missed. So long as your fish are healthy less food for 2 weeks will not be a problem.
Water changes should be done in the days before you leave and not on the actual day, on the actual day there is always a chance of problems, like forgetting to turn something back on. The amount of water changes will depend on your water parameters, you want them all to be at acceptable levels the evening before you leave.
As soon as it's convenient, unless the fish are showing signs of distress then ASAP, after you return a 20% water change should be made.
Are you showing your fishsitter what needs doing in the time before you go?
Any dried foods could be prepared and put in something like a daily pill organiser, which will also reduce any danger of overfeeding.