Window (not full screen menu, etc.) over parent window (as WinForm) with full autiscalling depend of screen size with resize bacground photo of this parent window to size of this parent window

Hello,

After some learming and practical experiments i do not sure what i know most optimal method for following simple and base action..

Unity 5. 3D game.

I have main menu first page of game with bacground image. "Screen Space - Overlay" is working fine (excluding text proection - it is look like no perspective in difference with "World Space" variant) I have tryed "World Space" but in this variant bacground image have some problem with auto resizing depend of screen size (or window size) - sometimes it can be cutted sometimes is smaller than need.

1. Is there optimal method for resizing bacground image to full window (screen) size (without cut it and without not resizing it to al corners of window (screen)) ?

2. If i need to open chield "window"-screen (as example - chield menu) with smaller than base window size over this parent menu (scene) with all described above autosize features - How to do it by most optimal way ? (There are some methods but i not sure which from it is optimal.) It is better if this parent "window" can have management of transparency..

3. Just have appeared another base "question" - Unity is support 1 window (with ability of many virtual "windows") or it can be multiwindow (i.e. with real multiwindow mode) ?

Best regards, Oleg