Hi Ralph Tube renewals on US stream locomotives are every 5 years of regular service. Heater tubes might last longer as there is no corrosive smoke. Cleaning and leak repairs would of course be more often.
The space for removing the tubes is figurative, it shows the length of the tubes and their projected end area. In reality they are removed one tube at a time and that allows a degree of turning as they come out. The heat & cook heater drawings show that the tubes are in horizontal sub-sets. Working from the inboard end, if all are being renewed they would come out from the top down in each sub-set leaving the space above the set as room for tipping the inner ends up as the tube tilts down to comes out. If just one or two tubes need to be replaced it might be more difficult or require a few extra removals. An additional help is that the bottom of the separator, below the inlet pipe, likely comes off.
H&W probably didn't correct Weirs about an intent to renew the tubes from the inboard door instead of the outboard one. I didn't even see an end specified on the H&C heater.
I take it Karol has seen the H&C heater overview drawing posted here a few years ago but I don't know if he and Steve have seen the whole set of boiler, heater, filter and evaporator general drawings dispersed throughout the British National Archives/Board of Trade file, MT15-212. So I will mention that a large set of files can be downloaded from for US$8 at http://www.paperlessarchives.com/titanic_disaster_british_natio.html . Their Titles button lists some other combinations of Titanic documents too.