I don't have the answers for your other questions, but as far as taping over the barcode, that is perfectly OK. You will not have any problems doing that.
Think about it, USPS (like UPS) has clear envelopes you apply to the box, and slip the labels into. The scanner has no trouble reading through that, and the label isn't even in contact with the clear plastic, so the laser can be distorted as it passed through. But it works fine.
When you apply tape over the barcode, it's in direct contact with he printed surface, no problems with distortion. Reflection is also NOT an issue.
I think the reason they say not to do this is the possibility exists (or did exist at one time) for certain inks (ink-jet type inks) to smear or bleed when in contact with certain adhesives from labels. With a laser printer that isn't even an issue. smearing or bleeding could cause them to become unreadable, but as I'll explain below, it doesn't matter.
My advice to you is tape over the barcode. Make sure the tape is smooth and flat and has no crinkles when you apply. if you get a bubble, pop it with a pin, and press it down.
Not having tape over the barcode means the barcode can get ripped and torn or, or defaced and made unreadable anyway from scuffing other boxes.
Just keep one thing in perspective. The barcode alone is not what's being used to move your package to it's destination, the address is still the default. The big barcode at the bottom of the label is a 2D barecode with the delivery confirmation number. The small square barcode (or "dotcode") near the top is a 3D code that contains routing information for automated handling. If that code became unreadable, manual handling would be required using the printed address.
So tape them over, and stop worrying about it!