The short answer to most of your questions is : only when needed .
1. In northern climate , Sept 1st is usually the best time to plant grass seed .
2. NEVER "scalp" your lawn . Common bluegrass/fescue type lawns should be cut to 3" . This will minimize any need for watering and help control weed germination .
3. Unless weeds are completely out of control , spot application of a weed killer (2-4-D type) is usually advisable .
To get specific answers for your lawn , your area extension service is a good resource . Hard to give specific advice w/out knowing any particular issues .