Intramuscular is very different to intravenous, and doesn't seem to have been explored much for pentobarbital. Plenty is known about other drugs though, like benzos, ketamine, etc.
"Pentobarbital can be administered intraperitoneally or intravenously. When used as a general anesthetic, pentobarbital can be administered as a bolus (Borkowski et al., 1990) or CRI. Commercial solutions commonly contain propylene glycol, which can cause pain upon injection and thrombophlebitis. For this reason, the subcutaneous and intramuscular routes of injection are not recommended. "
www.sciencedirect.com
This kind of literature is designed for doses that are tiny in comparison to a lethal dose; these could be administered in one small IV shot, while the 12g dose could not.
The commercial bottles of veterinary pentobarbital contain propylene glycol, mentioned in the quote. 100mL is a very large amount to inject, and it's theoretically possible to do, just very difficult unless you are trained in doing this. The onset of most drugs through intramuscular injection is 5-15 minutes, compared to seconds for intravenous. I'm not convinced that gives you enough time, since intramuscular injections are slow and you'd possibly need to cycle through different muscles (maybe have like 5 syringes ready to depress) to get that much inside yourself in a short period of time. Intramuscular gives you more time than going straight for a vein, but getting 100mL in your muscles in such a short period of time is going to require a lot of practise and anatomical knowledge. Technically the drug can be administered intramuscular, intravenous, by mouth, and even rectally, but there's not much reason to pursue anything other than drinking as liquid. IV could work with a slow drip, but you could dislodge the needle or do something else wrong. You're not going to inject 100mL in one plunge of the syringe without falling asleep before it's all in, unless you inject so fast that you destroy the tissue painfully.
As for the question about bags, my curiosity here is whether using one would actually make the N more likely to fail. N needs time to absorb to a lethal dose, and a small bag will run out of oxygen before that time has fully passed. Vomiting could be a result of oxygen deprivation/suffocation, and you could vomit up the liquid which hasn't made its way into your bloodstream yet. A bag with time-delayed flow of nitrogen would be another story though, and the ideal would be to take N then have your oxygen removed after an hour to ensure death, although this is overkill for most people but would be the kind of thing I'd hope to see in medicine in 20 years.
It's good to be curious, and I'm not a medical professional, but it seems like introducing points of failure in the procedure that are unnecessary, and if you rig up some drip system, it'd be a lot harder for anyone around you to claim ignorance about your plans. As pure powder, it's a very small drink then you go to sleep about 10 minutes later, and die within the next hour or two completely unaware of suffocation. Apart from vomiting it's all guaranteed, and all the guides suggest antiemetics. Even cannabis would not interact with N or antiemetics, and would be fine to use for nausea on top of everything else. The drink isn't very big, less than a shot glass I think. There are ways to numb your mouth, tongue and throat too.