As your descriptions are provided, I can tell the main problems lie in the first two Windows ISO downloading and the file format of your MacBook to your USB drive. You can perform successful ISO downloading work again and launch your Windows PC to complete the formatting task.
You claimed that the initial two Windows ISO downloads were agonizingly slow and ultimately corrupted. You should thus visit the official Windows servers once more to get the Windows ISO.
Next, the drive formatting on your MacBook Pro running Mojave version 10.14.6 to your USB drive was likely unknowingly problematic. You can format your drive on Windows. Read on to format your USB drive on Windows here:
Step 1: Insert the USB drive into your PC gently.
Step 2: Launch Disk Management and locate your USB drive in it.
Step 3: Right-click your drive to select Format from the contextual menu.
Step 4: Set the drive information, including allocation unit size, volume label, and file system.
Step 5: Check Perform a quick format and click OK to begin the formatting process.
After the formatting procedure is done, work to give your USB drive a proper MBR is indispensable. Since you have windows data on your PC but cannot log in, you should head to Windows > Boot folders on your drive > run boot sect to create an available MBR.
Then, open the Windows ISO on your MBP and copy the files inside of it to your USB drive. And insert the USB into the PC and enter the boot selection menu and choose the USB.
Hope this helps with your problem!