How to use the Shutdown pin of a switching regulator?
2- What voltage would be on the OUT pin of MAX756 if it is shutdown? The same as Vin? or is it floating?


For this specific converterr, the output will be 0V. But there are other components (just as an example I mention the TPS61291) with a bypass mode, where the output is connected directly to the input when the converter is shut-down. This could be useful if the MCU could still operate with Vin.

As a general comment, to answer questions like this (I'm mostly referring to your point #1) you have to make your calculations and see how much energy will you save if you do this or that. For example, you want to have 100 Hz sampling rate, that means max sleeping time 10 ms, which of course you will not have. Let's say it will be 5 ms. So if you shutdown the converter you will save around (200 uA - 20 uA) * 5 ms * (100*60*60) (times/h)= 0.09 mAh pro hour. Just very rough  calculations here! If you think it is worth it, then do it, otherwise don't. Every design and every system has different specifications and needs, there is no general rule you can apply everywhere!

Also, something else. Have you considered the startup time from sleep mode of the MCU and how much time it will take you to sample, process and transmit your data? It may not be possible to go into sleep mode at all with this sampling rate you are talking about!