1) To make the tank in metal : fit formers to the spectacle plate and smokebox front plate, aligned on each with wire thro holes. Tin the edges of the formers before fitting as well as the edges of the tank. Mark on the waste of the etch where the sharper bend need to be. Bend the tank to suit, large radius first and the solder it all together. There are some issues with the riveted straps but easily dealt with.
2) 3D printing is fine for small part patterns which can then be tidied up. It doesn't seem to work too well for large radius bits tho' (been trying to produce clerestory roofs)
3) Drive the rear axle and make the lower half of the boiler part of the chassis ? High Level Models of the Neilson and Black Hawthorn do this. You do not need a large motor. Both my HL kits will pull 10+ wagons more than enough for this size loco have no extra weight in them and a 1220 motor and the wagons are all kit built.
4) I think only 3 crane tanks were built by the LNWR and their main duty was in the works. They were certainly not trip working locos (fancy firing one for a 50 mile trip??) and would very rarely if ever be seen in local yards although they could have been found in docks where the 0-4-0s spent alot of their time.
That's my last words on the subject.
Nick