1) According to the Linus Pauling Institute at Oregon State University, if you are consuming vitamin C orally, whether in food or in supplements, you see 100% absorbtion of the nutrient at doses up to 200mg. Once your dosing exceeds 500 mg, you see fractionally less absorbtion of any vitamin C you consume past that point. Once vitamin C concentrations in your blood plasma reach max capacity, additional vitamin C is largely excreted in the urine.
True, but these studies were done in healthy young volunteers. The LPI recommendations, for example, are to maintain health - not treat disease (how do I know this? I helped write them!). We don't know what happens in people who are sick. While not the most reliable source, Robert Cathcart did show a very unusual phenomenon of increased bowel tolerance for vitamin C during illness. To my knowledge, this has never been adequately followed up in clinical trials.
2) IV administration of vitamin C bypasses this restriction and allows higher doses of the nutrient to be sustained in the blood. Historically, researchers have proposed that vitamin C supplementation, in general, could be used to treat pneumonia because the condition is a common complication of scurvy, which results from vitamin C deficiency. Since then there have been anecdotal accounts, case studies, and preliminary clincal trials attempting to use IV vitamin C to treat pneumonia, with mixed or ineffective results (see here). In other cases, when vitamin C appeared to help somewhat, I've seen that the treatment is sometimes combined with other interventions, so it's difficult to discern the effect of vitamin C in isolation (see here).
Because intravenous vitamin C does allow for high amounts of the vitamin to enter the blood, it can have different effects than high supplemental doses. Intravenous vitamin C - for example - it thoughts to act as a pro-oxidant in cancer patients.
As for supplemental doses of vitamin C in pneumonia, Harri Hemilä did a review of that topic back in 2013.
3) In the context of this study, shared by @vedhogger, the IV vitamin C they're describing as "high-dose" is only a 50-200mg dose, which seems in line with standard vitamin C supplementation. In addition, they are recommending that course of treatment when treating patients with mild or no symptoms (assuming my Chinese to English translator is accurate). In other words, in this context, it seems as though the treatment team is ensuring that patients receive enough vitamin C to maintain normal immune function.
This link is recommending "vitamin C is administered at a dose of 50 to 100 mg/kg per day" for light cases and up to "100 to 200 mg/kg daily" for those experiencing a cytokine storm. If we assume that that average male weights 70-75 kg, that's talking about doses up to 15 grams per day.
However, on this point I do agree with you: a recommendation by a medical association does not mean the treatment is working. YouTube videos do not count as evidence, nor do "press releases" by the Orthomolecular Medicine News Service