In regards to Q1, it sounds to me that permissions are locked by administrators at the project level. A server or site administrator can do this when going into the permissions of the project:
You too are correct that enabling/disabling downloading summary or full data can be done using this window. You can learn more about permissions here: Permissions Reference - Tableau
Speaking of learning, there are many resources out there, but you've already started on what I think is one of the most valuable: the community. Try to learn as much as you can from other's questions and your own here. But there are a few other ideas that I typically suggest:
2. Look for specific's topics reference in the documentation or simply in Google. For example, if you search for LOD: Overview: Level of Detail Expressions - Tableau
3. If you are going to be working with Tableau on a regular basis, I'd encourage you to take formal training. A really great place to start is e-learning: eLearning: Tableau Web-Based Training
Hope this helps!
Thank you so much, Santiago.