He could not figure out how to install the GPU on the MOBO and said its incompatible
This was the first red flag that he didn't know anything about building pc's. If you are building a new pc, the gpu uses a pcie connector and it exists on pretty much every single motherboard for the past 10-15 years now, maybe even longer. The only incompatibility that the gpu can have is with the psu, but not with the motherboard.
he said maybe ur PC got overheated and hence GPU wasnt spinning
Gpu may have a hybrid mode where it will not spin if there's no load.
How can my PC overheat if all I am doing is installing basic softwares..not even installing or playing games
Apparently, it can, while no one pointed this out, I'd still like to point out that under the cpu cooler, there is a plastic film that needs to be removed before installing the cooler. There's a possibility he did not remove that. Or the cpu cooler was installed without thermal paste.. In these 2 cases, the cpu will overheat even without a load.
(a) Can I get a new glass for this MSI case - MAG force 120A
Just contact msi, it will take a few tries, I purchased a pc from someone of te and the front usb port came broken. It was nzxt case, out of warranty, I had to create 4 different tickets with nzxt but one of the reps agreed to help me and contacted the supplier/support team in delhi, I got a call and he just asked for shipping, I got it picked up myself and the panel came for free (for a case that was out of warranty, and discontinued) so I have hope that you will get a replacement panel.
(c) How do I know if he has given correct amount of power to my GPU..how do I check that
You cant decide how much power you give to the gpu, while there are apps like afterburner that help set the power limits, the gpu will work fine without any such utility.
(b) How can I check if he has installed the CPU and GPU correctly . The PC case itself has six fan..plus one CPU cooler fan and GPU has two fans..with so many fans how can my PC overheat? what tests or benchmarks do I run to check this?
As you mentioned someone contacted you and is willing to visit you, someone also texted here that he's willing to visit and fix it for you, I'd recommend wait for someone, but sit with the person once he opens it up clear your doubts as you guys proceed.
As for whoever is willing to visit the op, I'd recommend atleast take a tube of thermal paste along with you, if you have one lying around as it possible that the cpu cooler was installed without applying it.
Peace
And he ended up taking 2K
Plus, if you can, take that money back.