Can stored/leased 737s be used to substitute for grounded MAXs?
It really isn't 1 new aircraft = 1 mothballed aircraft. Most of the 737 MAX aircraft were fleet expansions (for US carriers). The older aircraft get retired when they aren't economical to fly anymore, and even then a retired aircraft is often sold either to a scrap yard or to a lower budget operator. In order to bring an aircraft back into service they may have to go through C or D checks before they can be considered airworthy again (because old aircraft are often retired at a D-Check when it needs a major check). 

Airlines need to schedule C/D checks out months in advance, each aircraft costing millions to go through the check and a month or more of downtime. There is only limited space available to do these checks, so even if they wanted to bring 10 aircraft back into service, it may take 6 months and cost $20-40 million. It's cheaper to cancel the flights.