Lots of issues in Inspiron 15 7559 even after repair

I have Dell Inspiron 15 7559 laptop which I had purchased in Jan 2017. It had windows 10 preinstalled on it. Got the windows 1607 update and it worked fine for a few days like till February first week. One day the windows login screen was just stuck on date and time screen. It wasn't letting me enter the password. I contacted Dell regarding this issue and they sorted it out with system restore. But this is just the beginning. After 2-3 days the same problem came. And Dell told suggested me to reinstall Windows and they guided me on how to do the same. After updating windows once again the same problem related to password was back again. Now Dell gave up and told me to contact Microsoft saying that it is a Microsoft issue. I was on call for 3 hrs with Microsoft they tried everything, even took remote access in safe mode but nothing seemed to work and they told to reinstall Windows. And this process was occuring repeatedly. Then some new problems started to occur. First was related to task bar stating that there is critical error and task bar was stuck with a message that sign out so that windows will repair task bar. But it didn't do. Had to reinstall Windows for the same. One wierd error was related to BIOS. I turned on my laptop and bios was asking me system password followed by HDD password followed by admin password which I didn't even set. So it was actually a wierd and known error in Dell laptops which I got to know after searching for this issue. I have literally reinstalled windows 10 for 8-10times in February after the problem started. And the icing on the cake was the blue screen of death which came twice before I sent the laptop for repair. They replaced motherboard and hard drive in the laptop and I got it back from repair centre on 23rd march 2017. The disk usage in task manager was stuck at 100% after I got it back and problem related to Windows is still there. It updates and it doesn't accept the password after the update. I have tried everything right from installation of Windows 10 with a new key which I bought from my University ( I am a student and fed up of this as I have already lost my labs and assignments in it), different ways to sign in into windows, changing windows password, trying to start in safe mode and all. I have already tried to reinstall Windows once again but guess what now it is not even installing windows and gives an error that windows did not finish installation and then restart your installation (already tried 3 times and same error occurs).I want refund as it is just wasting my time on this useless laptop.