1) Go to Tools -> Options -> Editing -> Editor Windows, and change "Files per window" to "One file per window". When you initially switch to Brief emulation, it changes this setting, but you can change it back. All the windows you open after that will show up in their own tab.
2 & 3) There are a lot of ways to start search and configure how it works. For this, I'm assuming you're hitting F5 to search and you're getting mini search window that's in the upper right of your source window. Let me know if that's not right. For that mini search window, selecting the little ".*" button will enable regular expression search. And the mini search window can be configured to close when you hit the enter key on your search expression, if you go to Tools -> Options -> Editing Search, and under the "Mini window options" turn on the "Close on Enter key".
The other brief search method that doesn't bring up any dialog is Alt-S, which prompts for the search string in the message box, and you can toggle regular expression search with Ctrl-F6 for that method. You just have to hit return to start the search, no window to cancel out of.