Anyway, addressing the first question in your list (no way of knowing correct dosage for IV administration compared to ingestion), the report does have this to say, (my emphasis)
"Sodium nitrite administration has been reported to cause or significantly contribute to mortality in adults at oral doses as low as 1 g and intravenous doses as low as 600 mg. "
Does that not indicate equivalence between an intravenous dose of 60% of the ingested dose ?
It's pretty obvious that a self-administered lethal dose of SN cannot realistically be delivered intravenously.
Recommended ingested dose = 25gm
60% of that = 15gm
SN in liquid form contains 300mg SN in 10ml water.
That means that you would need 50 liquid doses @ 10ml, or half a litre.
So this is just from my research, and sometimes my research just does not stop with articles that have sodium nitrite as a subject, the research also has to branch out into other things. For this I had to look at the difference between the ingestion orally of a drug vs IV. Human metabolism and the process of toxin elimination via the digestive system. The answer is yes, I do have a lot of time on my hands.
When ingesting, the liver has a good crack at cleansing toxins before it reaches the bloodstream. The stomach does not transfer consumed items into the bloodstream from my understanding, it is the small intestine that does that. So if you take a dose of say 15gm of SN, the digestive process will eliminate some of that perhaps before it gets to the blood. Taking something IV, your system will assume that you liver will have had at least a passing glance at it and just circulate it around the body. So the time difference for ingestion vs IV is pretty obvious.
The lack of liver activity for IV also means that technically you need less weight of SN to do the job. The SN vials used in cyanide poisoning don't come in mg but volume percentage in a solution/liquid. Obviously with some maths you can work out what that actually means in weight if so desired. Advisory notes that come with the SN vials that death
could come after two 10ml doses (20ml for those who are tired of doing maths). However, we then need to factor in natural tolerances and the ongoing blood cleansing process that the human body has, each of us are different. That's why I took a guess and suggested 40ml to achieve the job as to overwhelm that cleansing activity.
There is also the condition of the vein/muscle to consider. When doing cyanide poisoning resolution, the SN is practically trickled into the vein rather than just shot in. Can't find the reason for this anywhere. The only clue I have was with
@gingerplum when I first muted this. Varicose veins are treated with salt injections to close them up. So perhaps just shoving the hypodermic needle in and shooting in a load of SN quickly would do serious tissue damage and in turn prevent getting the SN into the bloodstream. That is pure speculation from me rather than fact.
i am not a real doctor, I just play one on TV.