Rear ported speakers

Itll cause the bass frequencies to sound boomy and exaggerated. if you have to have them up close to a wall of in a cabinet, its best to use the foam port plugs, but that will limit the low frequency output.

I've got the same issue. I have to put my speakers in a bookshelf, so I'm limited to front ported or sealed, which significantly limits selection. I'm leaning toward NHT.

Damaged? No.
Sound absorbing panels are a good idea regardless where you place your speakers. Bass traps are a good idea too. Proper speaker placement is another good idea. Lastly, looking both ways before crossing the street is encouraged.

Only if it's so close that the space between the wall and cab becomes in effect part of the cab, and even then that's not necessarily a bad thing. Varying that distance can fine-tune the cab low end response to the room.

Not necessarily. With the right distance it can extend response lower while taming boom in the midbass.

It would have to be very thick to have any effect on the low frequencies that radiate from a port. It would have more effect on the diffraction of higher frequencies that come off the cone, wrap around the cab and the reflect off the wall.

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