Eventually I got it working for my Brother DCP-T420W, more or less. Fo those who are still facing this issue, Below is how I got the drivers working.
- Uninstall all brother software including iPrintScan and drivers from control panel and use Uninstall tool from brother website to remove the drivers.
Link to uninstall tool: https://support.brother.com/g/b/downloadend.aspx?c=eu_ot&lang=en&prod=dcpt420w_all&os=134&dlid=dlf005059_000&flang=4&type3=39
- Download and install iPrintScan
Link to iPrintScan: https://support.brother.com/g/b/downloadend.aspx?c=eu_ot&lang=en&prod=dcpt420w_all&os=134&dlid=dlf103486_000&flang=4&type3=10272
- Once iPrintScan is installed run it and it should not show any machines (donot add machines just yet)
- Download the Printer Driver from Brother website and install it. It should be a quiet install but you can modify the "dpinst.xml" file and remove quietInstall tag to get the wizard appear.
Link to Printer Driver: https://support.brother.com/g/b/downloadend.aspx?c=eu_ot&lang=en&prod=dcpt420w_all&os=134&dlid=dlf105100_000&flang=4&type3=408
- Once printer driver is installed download "Full Driver & Software Package", double click it to get the files extracted into "Y20A_C2_UW_PP-inst-H1" folder but cancel the installation.
Link to Full Driver & Software Package: https://support.brother.com/g/b/downloadend.aspx?c=eu_ot&lang=en&prod=dcpt420w_all&os=134&dlid=dlf105099_000&flang=4&type3=11
- Use 7zip to extract "Y20A_C2_UW_PP-inst-H1\Msi\ScannerUtilityInstaller.msi" and you should get a new folder within the Msi folder named "ScannerUtilityInstaller" with a file named "BRSCUTIL.EXE".
- Run the "BRSCUTIL.EXE" and select the correct dpi (for me it was 1200x1200)
- Once the installation is completed you can add the Printer/machine in iPrintScan (Make sure it is connected via USB) and you should have your printer and scanner working in iPrintScan.
Note: This process has only installed the printer for USB connectivity. Later I tried to add the network printer after configuring the printer through it's management page and connecting to Wifi but the Print service does not work. If anyone know how to get the Print service working in iPrintScan please share.