I don’t know how the US equivalent of UK Listing works (is it National Monuments, or is that the next tier up?) but my understanding is that here each building is judged on its merits; the architect would be taken into account, but wouldn’t be the sole criterion.
My sister happens to own a cottage designed by the same architect as the Dorchester Hotel in London; the cottage itself isn’t remarkable, but is Listed because of the architect.
Yes, but it’s a big deal; application to central government via the statutory bodies rather than a local official.
Depends what it’s Listed for. If it’s solely for the exterior, modifying the inside would probably get approval from the local planning office without hesitation, but if the interior is the important part, don’t even look at it funny. 
I frequently see a Listed facade propped up by scaffolding whilst an entirely new building is constructed behind.
Sometimes the rules are treated… creatively. This is an example local to me. The Listed part is the roadside facade of the grey-roofed retail unit plus the low building under the orange ‘U’-shaped one.
Here’s another; apparently it’s good enough to meet the regulation!
