Brian,
Are you aware that most interior fittings are usually completed by the time the ship is launched so it is ready to be fitted out as soon as possible? I assume that Titanic's and Olympic's fittings would have been completed almost simultaneously. Also, I doubt that such a show piece as the grand staircase would be left unfinished or carved simply just to save time. If most paneling was not complete by the time Titanic was launched, it was at least complete once the ship was ready to have her passenger and crew accommodation to be fitted. There would be minor changes that were done late and quickly, such as the Cafe Parisien and the two cabins in the aft staircase, other late arrivals would be various furniture and other fittings such as possibly the clocks. However, Titanic had a master clock that controlled about 50 clocks all over the ship - which I'm sure all arrived to the ship at the same time. This included the 10 or more clocks throughout the grand staircase areas (fore and aft), lounges and smoking rooms, crew areas etc, etc. Why one clock in the forward grand staircase was forgotten seems a little odd. If a mirror was in its place, perhaps it was only temporary for whatever reason, and was installed once the ship was in Southampton.
Daniel.