For question #1:
The laptop monitor will be interpreted by device designer as a Tablet. That is because the smallest dimension (600 pixels) is less that the >800 pixel breakpoint that defines a Desktop display. Read the last portion (testing your dashboards) of our online help for details:
So in this case, adjust the Tablet layout using Fit Width or Fit All sizing to ensure that the dashboard is readable. You can also design a dashboard that scales predictably across Desktop and Tablet. That is, one that just works across those devices without significant changes. I can send you an example.
For question #2:
You can only apply formatting changes in the default layout. Think of default as a master template that controls worksheet formatting and actions. It also determines which worksheets appear in the device layouts.
Let me know if you have additional questions.
Product Manager, Tableau Dashboards Dev Team
Regarding # 1. As long as my device has a similar resolution to the tablet, Tableau won't differentiate about it being a laptop monitor or a tablet. Is that what you're saying? It would just know by the resolution which layout to use. That's great. This helps us solve the issue when client projects using really low definition projectors. We were concerned about the lack of a "projector" layout.
Regarding # 2 Let me see if I got it. For elements that need different formats (like font size) depending on the device, the suggestion would be to add the same element three times with the different formats to the default dashboard, and then, use them respectively on their device. Would it impact somehow performance? I am guessing any action changing the dashboard would require the three layouts to be refreshed. I don't want to think about some workbooks we have that have 5 analysis getting refreshed at the same time since they all share filters. That would mean 15 dashboards being refreshed at all times.
Thanks for your answers, they're really helpful!
For #1: Yes, Device Designer is only detecting the embeddable space in the mobile browser window. That's it. It does not differentiate between device types/models. As for #2: If you need very different formatting between Desktop, Tablet, and Phone, you can duplicate worksheets and modify their formatting as needed in the default. The key is to create all three layouts, so that default behaves as a scratch pad or staging area for planning the device layouts. This 'staging area' approach will not impact Server performance. It won't result in slower download times or viz interaction performance. However, when you create dashboards with Device Designer, we recommend that you use Fit All or Fit Width (automatic sizing). Dashboard automatic sizing works better for phone and tablet devices as it allows the dashboard to scale across a variety of screen sizes. That said, fixed size dashboard have a higher likelihood of hitting cache (better perf) because all requests will ask for the same embeddable space. So there is a trade-off when using automatic sizing.