validation and reserved seats

4 of us going to Japan In august. 1st week we staying with friends and not a lot of travel. on Tuesday of second week we are going to Kyoto from Tokyo for 3 days. Will return to the uk the following Monday. So if we activate the pass on Tuseday before Kyoto trip will it still work on the trip back to narita on Narita express? And I heard its very busy the second week on the shinkansen so will need to book reserved seats. If we change the order for a pass when we arrive in japan are we then able to reserve seats on the sinkansen even if we are not to validate the pass until the second week?