No liquid nitrogen would do the exact opposite of help layer adhesion. The issue isnt getting the hot material cold its getting the already deposited material hot enough that the new material being added causes a "skin melt" for bonding.
While Vacuum would be superior....a proper vacuum chamber and pump is a bit pricey to test a concept.....Argon or helium would be better choices....Helium youd just have to put the printer in an inverted box, run a tube, and crack open a balloon tank from any party store....Myself Id go argon....being heavier than air its really easy to contain, maintain, and control the atmosphere....and its terribly easy to comeby. Shielding would be a less significant issue in materials less prone to oxidation than aluminium.
The wire fed welder idea was being pitched as a form of deposition though it could serve the dual purpose of robotic welding.
Okay so assuming heating elements are your final choice of attempt....my take on how to go about it.....
The duty cycle on the nitride elements you cited appear insufficient for 3d printing.
you could perhaps machine a steel "pen" tip with a body wide enough to accommodate cartridge heaters.....
if you are only using solder than you can likely use a reprap style feed with a few modifications....all metal of course and you may need some spring tension to maintain proper pressure on the solder. The hobart spool runner is only a couple of hundred without the welding supply....so if you decide to go aluminum it might be worth hacking the spoolrunner gun as it already feeds aluminum wire from a spool....just a thought.
Now when it comes to eutectic alloys Id avoid woods metal....so much lead. There are plenty of lead free alloys to choose from.
Ultimately if the properties of these metals are acceptable to your intended applications....Id sooner print in plastic, refine and polish by hand, make a silicone mold, and cast as many of them as I could ever want. That is after all the great advantage to these alloys... their temperature is too low to require using sacrificial patterns and investment material...they can be cast repeatedly in rubber or silicone molds.
Ill not comment on the build detail of your delta as its still far to theoretical to have a known robustness requirements. Ive used a delta 3d scanner...one of these http://youtu.be/KA-oEvmXlfk?t=14s
from the point of novel utilization of an existing structure (in the case of your printer doing double duty) its a great idea....but they are very slow in comparison to structured light or photogrametry....and they (deltas) are unsuitable for capturing some geometry.
I would say that a dlp photopolymer printer capable of directing the projector to a second scanning chamber using structured light would come closer to your Ultimate first gen replicator than what youve described....but that is just my take on the subject.
and as for your "rambling"...no need to apologize this is your thread....ramble on..