The OP was discussing IV specifically, not IM, so let’s put IM advantages aside for this discussion. I’ve never done IV or IM anything , so I’m basing my opinions primarily on what I’ve read – Strassman in particular.
To address some of your points:
I don’t think the dosage differences are significant enough to justify IV over vaporizing. When properly vaporizing, maybe a milligram is lost to condensation, another couple of milligrams lost when exhaling, and maybe a small amount remaining in the vaporizing device. So IV requires a few milligrams less material to get similar effects. From the IV descriptions in Strassman’s study, it seems that the come-up and length of experience is very similar to vaporized.
There is little to no lung discomfort when DMT is properly vaporized. In fact, if it is vaporized slowly and gently in a GVG, it feels like warm air is being inhaled. With proper vaporization of relatively pure DMT, only DMT vapor is inhaled.
The “sterile technique” you quoted is very thorough and obviously necessary to minimize risks, but how many users are going to follow this procedure? And how many are experienced and skilled enough to follow it correctly? There are many opportunities for error, hence disadvantages.
(My sister had a friend who was an IV user of heroin. She died from liver failure due to Hepatitis C.)