“ I suppose there could be some sense of pressure to sway tests, etc. (primarily with dirt work).” – If you mean that the way I read it, I find that to be offensive. I am not sure why you imply that Geotechnical Engineers would believe they should “sway tests” more than any other engineer.
Do you mean that there would be any less pressure to let the thickness of fire proofing slide, or not to re-weld a series of connections that did not meet spec, adjust rebar clearances, etc? It seems that there would be many more structural items, compared to a few passes with a soil compactor, which could impact the bottom line, and your share of the design/build profits.
This is why the engineers are often put in charge (by law) of the D/B team, because they (should) have a higher ethical standard. The way I see it though, if the engineer is the lead entity, the owner has hired them to provide a complete project. The engineer has hired the contractor (putting aside contract specifics, as the engineer is the lead). Therefore, the engineer is technically working for the owner, and should be able to do the work, as the building official has pointed out. After all, you are putting your career on the line by signing off on it. I do feel that there is more pressure on all aspects of the inspections to be sure the contractor (sharing profits with the D/B team) is making money. This does create some conflict of interest, and should be watched.
My previous company had provided SI overview on a fairly large project (not D/B). The owner knew we would be too expensive to do the entire project, and also felt the could be conflict with us providing the SI and designs (another topic). Since the forgot to hire someone to do the SI, we did the initial soils work, then hired another firm (based on bids and quals) to do the remaining. We also provided review, compilation, and submission of testing results. I think as an owner, I would like that situation for a DB. Let the D/B team provide the inspections, and have a third party review on my behalf.