I use Civil 3D, current version, and work for an engineering company. I have little experience with Intellicad, and used Carlson for about a year while working for a surveying firm a few years back. These are my thoughts.
LandXML is the way with regards to sharing things like surfaces and alignments. Others may disagree, but I wouldn’t bother including the ASCII file of the points. The engineering company will probably just take your XML file and create a surface directly from that. Unless there is some reason that they would want to revise your surface, the points are not needed. Also, unless they have a description key set that matches your descriptors EXACTLY, the points will just come in unsorted and would just become filler unless there aren’t a lot of points, in which case maybe they could edit the ASCII file to match their description key set, assuming they have one. If they ask for a point file go ahead and give it to them if you want, but it’s probably not a CAD tech asking for it, unless he’s a glutton for punishment.
I’m not sure how you’re getting Carlson to convert C3D objects, because to the best of my knowledge, no other software should be able to read them. Not even plain Jane Autocad can view them without object enablers, and I’m not sure that enablers are available for Intellicad, and even if they are, that only helps to view them, not to actually be able to use them for anything. I would ask the engineering firm to export their drawings. There are at least a couple of ways of doing this, including using the Export Civil 3D drawing command, and using eTransmit, which gives you the same capability. What this will do is it will explode all C3D entities so that they are no longer smart objects. You should then be able to open it directly in Intellicad. Then request XML files for smart objects such as surfaces, alignments, pipe networks, or whatever else you may need. I never used XML files with either Intellicad or Carlson, so can’t offer suggestions on that end.
One other thing I would like to at least point out. Nobody really compares these XML files. I’d like to think that everything is all fine and dandy while sharing information this way, but to be honest, I’ve never known anyone to actually compare the information. I point this out because recently I shared an alignment, with the same surveying firm I used to work for, and when they sent back a topo survey, alignment included, the alignment did not exactly match what I had. Not off by much, maybe a couple of hundredths, if that. Did things go south in conversion to Carlson, or perhaps in the conversion back to Autocad? I have no clue. I knew our alignment was the correct one, and I didn’t really need to use their version of the alignment, so I didn’t spend two seconds thinking about it, but things like that do concern me slightly.