re: Bench design

Wood movement is only an issue if you have grain on 2 pieces that runs perpendicular to each other like the end aprons relative to a table top.  If you have the grain on the top and the shelf run the same direction, wood movement is not an issue.  The top and bottom will move about the same so it won't matter.  If it was wider across the grain (wider lengthwise doesn't matter) AND the top and shelf were different species of wood, you could see a difference but even then it would have to be pretty wide for movement to cause a problem. 

BTW, if you want to learn Sketchup, this set of tutorials by Mattias Wandel really helped.  He does a good job of showing just what you need to know to get going and it is geared towards woodworking.  He uses a really old version of Sketchup so to use the current free version which runs in a web browser, you may have to search around for where things are but the concepts are the same.   The last free version that runs on your computer is V2017.  It will be very close to to what he uses in the tutorial.  I may still have a copy of it on my computer so PM me if you want a copy and I will look for it and share it with you.  

EDIT to add:  Probably only the first 5 or 6 tutorials are relevant to the types of projects you do.  You can probably skip the ones after that unless you just want a little more information.