Should I keep fast-boot, and secure-boot turned off, even after ubuntu installation..If I turn it on, after ubuntu installation, will that cause some new problems..
This does highly depend on your configuration and it's hard to say from remote. What you could do is to test this early so that you already know. If it doesn't work you might be able to switch back and your system still runs then. If there is anything of value on the disks — as usual — restore from your backups if anything goes wrong.
In windows 10, i had mapped power button, to Hibernate-system, should I change that?..to shutdown!... I use Hibernate a-lot, Even after ubuntu installation, will I be able to Powerbutton-Hibernate windows safely, without causing any boot problems...
Again, that you have to try, however I see no reason why that part of the configuration should have changed. Try it out and see how it works after the setup.
In ubuntu installation setup- Options under-Device for bootloader-installation gives many other options beside windows-boot option, Is it ok, to install on other options, as not to currupt windows bootloader.. or recommended to install on windows-bootloader .. Is there any other recommended bootloader settings.
That depends on your needs. You should use the recommended option for the version you install, when in doubt consult the resources of the operating system vendors (Canonical for Ubuntu and Microsoft for Windows) to understand the details for interoperability, the options you have and what the consequences are.
Will windows update as normally, or should I do some, changes before updating windows-- that is, im confused if windows update, - rewrites boot partition .etc such problems..
That depends entirely of the decisions by the manufacturer of that operating system as the update procedures may change anytime. For whatever confuses you with that operating system, you should contact technical support for any clarifications in regard to the update procedure as we can't speak for that vendor nor have any insights on that level of specific.
What is the swap partition that we create, during installation?.. are they used for recovery or some other ubuntu usecases...
swap means the exchange from parts of the memory (blocks of memory) from the random access memory (RAM) onto the disk (harddisk) and vice versa. The swap partition is the area on the harddisk (a partition is a continuous section of the media when addressed/accessed digitally) where those memory blocks are stored. This allows the operating system to offer more memory (as in RAM) as there actually is and works under the assumption that while some programs still reside in memory, they don't do anything or very little and therefore it won't hurt to slow their memory access.
So the swap partition is not used for recovery in the sense you likely mean but the use-case is how a Linux system works, as from the early day there was very little RAM available and therefore the default setup is with "a swap".
According to How to use manual partitioning during installation? "Swap is the partition for keeping unneeded memory pages, like Windows swap. Also it can be used for hibernation." Therefore if you're looking for such features/the ability to hibernate, double-check your requirements before removing the swap partition on your box. (I personally would not recommend to remove the swap partition on a standard box.)
What is perhaps useful for you and the many more questions you certainly have is to read and learn from the documentation for the version of Ubuntu you have in use (the official docs come per each version). You can find it as Hypertext Markup Language (HTML) easy to read with your internet browser here: https://help.ubuntu.com .
