What you want is called multi-session burning, i.e. adding more files to a burned, open disc. ImgBurn doesn't support adding files to that.
Windows XP and newer versions should be able to do that with their built-in features.
Booktyping DVD+R DL media to DVD-ROM is automatic for most burners (Optiarc included) and, as you can see...
... your burned disc was correctly booktyped to DVD-ROM, so you did nothing wrong.
Go to Tools -> Settings, click Restore Defaults and answer Yes (if you absolutely need to use custom settings then click No) and click OK.
This should restore the default path of the preview feature, so you can then choose the best layer break for your project.
You should be looking at the supported write speeds shown in the Media Information text to the right of ImgBurn's window, as those are the supported speeds by your burner/firmware/media combination.
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The latest firmware for that drive is over 7 years old, I think it doesn't support the media available on today's market, or the drive is just worn out. The best option would be to get a new burner.
On another (unrelated) note you should install SP3.
You may have a bad batch of discs or your burner may be dirty/worn out.
You could still try burning at 6x and 8x and try cleaning the burner with a lens cleaning disc.
If none of that works then I'd get a new burner.