Hi @russss ,
It still could be an internally damaged display assembly.
Try shining a torch at an angle close to the screen to check if there is any display visible when the phone is turned on. It will be very faint if it is there so try this in a darkened room to help see it if it is there.
If an image is visible then there is a backlight problem. If there is a display visible then try the touch as you may be able to work out where to touch and check if the screen changes etc.
All it means is that you know that there is less of a problem with the systemboard, especially if both the display and the touch were working, just not the backlighting. You would need to find the schematics of the phone to know which wire in the flex cable to the display was being used to send power to the backlights to prove whether the problem was on the systemboard or in the display.
If there is no display visible then the problem could still be in the display assembly though, as the previous owner did say that it had been dropped.
Whether the screen is visible or not when using a torch as a DIY repair all you can do is temporarily replace the display assembly and check the result.
If the new display assembly works then all is well.
If the new display assembly doesn’t work and if there was no image visible with the old display check if there is an image now with a torch.
If there is then you will have to check the systemboard to see if the backlight power fuse is OK. If there is no image then it is more difficult.
Try to source a replacement display assembly from a supplier that accepts returns, perhaps for only a small restocking fee so as to minimize the cost to you if the display assembly is not the problem.