Hi Dakshina,
Thank you for your real life testimony on using Linux in a professional world. This is what we need because at home, just to browse and multimedia things mostly work, but when on a professional setting things may change and when time equals money there is no time to fool around when people are expected to be productive.
This is one of the things that need to be addressed if we want the mythical "Linux year on the desktop".
(...)this LG Gram is my travel laptop, I badly need touch on this and that feature never worked, despite my efforts to fix. But I never tried Mint or Kubuntu in last 5 years. May be I should?
I think you should, at least, try to boot from an USB ISO live session and see if things have improved and became more likable. 5 years, on IT, is a lot of time and progress with new kernels, firmware and stuff. (LMDE2 does not support my newest laptop's touchpad, LMDE3 does [and I copied the file and it works now on LMDE2]). You can install newest kernels on old systems anyway.
But be ready for facing the new trend on touch and feel aesthetics: flat user interfaces where it's hard to distinguish buttons and active windows, LSD-friendly colors... It's easy to change theming but it might be hard to customize it according to your needs (mine, eyes getting old needing dark blackgrounds with green text the way we used to have eons ago

)
2. I really never understood how to restore from IMG file?
Something with grub or efi
At least here on LMDE2 fossil-land on newer hardware, grub-efi is needed.
In my opinion, UEFI is something that had too much push from Micro$oft and is not well implemented. If you're away from Windows, it can be a PITA.
Things were not well implemented and it needs a ~1MB partition on the hard disk to store the OS keys instead of a hardware chip on the motherboard. That partition has to be updated with new keys for Linux and that can be a PITA. If you lose that partition (and it's even in FAT with no journal!) your system might not boot and you'll have a little nightmare to get it to work back again and I believe it can even be harder in Windows.
I can't get GRUB from LMDE2 to boot on my newest laptop, I need an updated OS installed and it's grub works and allows me to boot into LMDE2. Days and days were lost around this...
Someone should write a solution in a step-by-step manner for this considering the combinations of efi settings in the bios, partitions on the HDD (or USB).
I'm sorry but we seem to be doomed here. There is not a proper UEFI standard followed across all the industry, so each vendor implements its one.
On my newest laptop it's implementation is chaotic. As I said, GRUB from LMDE2 doesn't work, and there are inclusively BIOS settings that will turn the screen and the keyboard off, bricking the laptop. After sending it to repair under the warranty and bricking it again just after receiving it

I called customer support and I was lucky the told me you need to boot with an external USB keyboard and blindly press a combination of keys so you can load the BIOS default settings and turn your screen and keyboard back on...
(by the way, the term BIOS is obsolete and deprecared, now it is UEFI but we get along with the old language.
The Portuguese say it right, BIOS 'stands' for Bicho Ignorante que Opera o Sistema (Ignorant Bug that Operates the System

)
My approach to backup and restore is copying files to hard drives using grsync (because copy can fail leaving incomplete and corrupt files!) and then mirror those hard drives. I've had quite a few of them dying out of nothing and luckily I have lost no important data.
Regarding a backup of the OS, I use fsarchiver on the terminal. There if a GUI for it that supports GRUB and UEFI, I think, but I do not use it).
Beware that when manually restoring a partition, you have to be
REALLY careful when selecting the destination partition, because if you use another partition by mistake (your documents!) that partition is wiped out for good faster than you can panic......
But fsarchiver, when dominated, is quick and easy. It compresses well (my backup files are between 900MB without wine and 1,5GB with) and they restore faster than going to the bathroom for a pee and washing hands
You may need to install or reinstall GRUB again, but if it's entries haven't changed it just boots (mine changes all the time because I use Liquorix bleeding edge kernel).
If I have to update GRUB it takes more time than restoring the OS, I have to boot from a USB live session (I always have one for recovery with the backup files on it) and then have to chroot inside the restored system (you hack inside your installation from the live session and you enter a root terminal where you can do whatever you want, in this case updating GRUB).
3. I have a lot of questions (may be very silly) on 'how to fix this or how to manage' etc.
There are no stupid questions, only stupid people. We all have our learning curve and that personal mountain to climb. This forum is the best place for help for Linux Mint. It's it's home, after all! I've been learning a lot here all these years!
Stupid people do stupid things. Smart people outsmart each other, than themselves!
(System Of A Down, DDevil)
You have fsarchiver's quick start guide here:
http://www.fsarchiver.org/quickstart/
and to install it all you have to do is
PM me if you decide to try fsarchiver and need a hand. Cheers!