By default, tabs are configured to only show up when the scene view is maximized. You can maximize the scene view by pressing Shift+Tab with the window focused, or by double-clicking it. You can also change your Peek preferences to always show the tabs, even when the scene view is minimized.

Toggle the first icon in the tabs toolbar to Pinned. Then, open your tab. This will let you move and resize it. When closed, the tab will remember its size even if opened as a popup later.

Yes. Peek even has a special ProBuilder integration with the Probe Tool.

However, the Peek shortcuts can sometime conflict with those of ProBuilder. For example, the shortcuts to navigate the scene toolbar, Alt+[0-9] conflict with ProBuilder's shortcuts to assign a material from the material palette. Starting in Unity 2019, you can use the shortcut remapper to resolve the conflicts.

In Unity 2018, you can comment the line #define ENABLE_PRIMARY_SHORTCUTS in ShortcutsIntegration.cs to disable the Peek shortcuts and give priority to ProBuilder. Unfortunately, because ProBuilder is distributed as a package, you cannot edit it to disable its shortcuts and give priority to Peek.

Yes!

Usually, the issue is that you inadvertently pressed B while in the scene view. Just press B again to re-enable the toolbars.

There are two preferences that control the display of scene toolbars: Scene Toolbars and Display Scene Toolbars. The former allows you to choose whether you want them never on, always on, or only on when in full screen. The latter is an additional toggle that can be toggled with the B shortcut in the scene view. Make sure both preferences are enabled.