The idea of making these cruise ships into command ships was never taken seriously within the Navy; it was just a bid by the lawmakers in the areas where the ships were being built to try to keep money flowing to their constituents. It just happened that the failure of the cruise ship initiative coincided with the Navy's proposal of a class of JCC(X) joint command and control ships to replace the current LCC and AGF command ships and some Congressmen saw a window to conflate the two.
JCC(X) also imploded due to budgetary shortfalls, but the cruise ships would never have been suitable candidates even if JCC(X) had continued (they had made no attempt at military damage control or shock-hardening, for example).
The hull of lead ship and materials for the second were taken to Germany, where the two ships were completed to different designs.
No, the SL-7s would make lousy command ships. They're not terribly reliable, not designed to house people, and probably larger than is really necessary.
Speed is not a compelling requirement for command ships. Since they travel with the fleet trains or amphibious forces anyway, a speed of 20 knots or so is perfectly adequate.
PS: I note that apparently JCC(X) is not quite dead yet. he Navy is apparently looking at adapted LPD-17 hulls for the role, beginning in the FY12 budget. This strikes me as a sensible idea -- LPD-17 is well designed for damage control and should be adequate in size (two of the four existing command ships are converted LPDs of older designs significantly smaller than the new LPD-17 design).