If you are able to avoid damage to the rear frame tube as you drill the rivets out, go for it... You can always replace the rivets with nuts and bolts.
Yes, you'll be increasing the rearward gain while reducing the forward gain of the antenna and as a result risk multi-path and other problems. You'll just have to experiment to see.
It sounds like you've already wrestled the multi antenna options to the floor... But if I was in the same situation, that would be an option I'd be looking at if the DB4e mod fails.
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