1. Since the movies didn't explicitly say "No BDZ", the BDZ stands. Besides, the guns fired in the movies are light TLs. The heavy TLs were fired only in ROTJ, at other ships, and at too long a range so see anything. An ISD has 120 light guns but only 6 heavies. The light turbolasers' yields have been pegged at anything from a few megatons to a few hundred megatons.
2. First of all, the ISD is 1.6 km, and second, that doesn't matter.
3. Same as #1
4. Same as #1
5. There are only six heavy turbolasers on an ISD, and again, the only time we see them, it's from too far a distance to observe anything, especially if the TLs strike shields.