Answers

If basement ceiling is unfinished or has drop ceiling you can usually run 1 1/4" screws from underneath.  It is best to have a helper finding the squeaks and try to measure from the outside walls in order to find proper placement of the screws.  Top nailing the boards is a temporary fix at best.  

With this scenario.  The least expensive start is to on make sure all gutters are free from debris and working properly.  2.  Remove sidewalk and re-install new concrete all the way up to the house and brick pitched away from the house.  After concrete is cured in a week have a contractor come back and put masonary caulk between house/brick and concrete walway.