Help bring my G3 back to life!!!

So, here's the story: I wanted to root my d850 on LP version D85021R, Tried all methods posted here, including this, this, and this. None worked. So I decided to downgrade to KK using tot/kdz method by hyelton. Downloaded LGD850AT-01-V10d-310-410-JUN-19-2014+0.tot file and LG flash tool 1.8.x and flashed the tot file. Progress went above 85% and then phone rebooted, but never came back to life; super-bricked, no led, no boot screen, no download mode, no recovery mode, NOTHING! except that it identified as QHUSB_BULK in Win7, which upon installation of drivers turned to qhsusb_dload 9008. Tried all unbricking methods including Board Diag, Flashtool ,etc. none worked. So I took it to a repair guy, who probably used octopus box to copy image from another g3 (at least that's what I think, he would not let me see but i sort of glanced over). After copying that image to my g3, it turned back to life, i.e., boot screen appeared and download mode/recovery mode worked, but it was in a boot loop. however, that guy also tried flashing V10D using flashtool, and same thing happened, i.e., super-brick again. After a day of trying he gave up on it but was kind enough to give it back half alive. So now boot screen appears, download mode works, gets recognized as LGE Mobile USB Serial Port, and LGE Android MTP Device with a yellow triangle. I cannot access fastboot mode due to locked bootloader, or else I would have tried flashing partition images through fastboot, as bootloader and boot partitions are intact, so I would not touch them but could try flashing other partitions. LG Flashtool flashes the whole package so the boot sector also gets over written, which somehow is not compatible with stock rom. One thing which I have not mentioned is that I did not buy it new but refurb one, so may be some part was changed, but which one, no idea. I tried to get it recognized it in linux, but does not work; shows up as libmtp-1-1 in /dev, which is not mountable. However I was able to pullout one interesting fact, which might give some hint. dmesgs shows a serial number for this device which starts as LGD855xxxxxxx, whereas the serial number printed on the back is different, where it shows as d850 and serial number is also different, i.e., 406KPXVxxxxxx.
Now I am out of options. Can someone please suggest what else to do. Also, can someone confirm that the serial number shown in dmesgs when connected to USB port, starting with LGD855xxxxxx can be a d850?
Any help is welcome :)

Been on stock for months, wonder what a Custom ROM will bring me..

Hi guys,

I have been on stock for months now (latest firmware, with factory reset aswell), and I keep bumping into people claiming a custom ROM should fix all janky stutters that Samsung applied as a feature to their software, same goes for the RAM management.
Weird, stupid stutters happen all over the device, even in stock apps like the Phone app (pulling up the digit input pad or switching tabs is just hillarious)..
It is really effecting my experience I have with this device, while it could (and should) be amazing, considering the power it has..

I've had a Nexus 5 before this, and things just seem smoothher in terms of scrolling etc.
Could a custom ROM give me the same non (barely) stuttery performance I had on the Nexus 5?

Sorry if this has been asked before, but I'm just very carefull with this thing as it might mean the end of my warranty.

What is your experience when you compare a stock ROM with a custom ROM?
I tried looking for comparison videos, but they are non existent.

Cheers!