I'm also aware that for some of the things you've raised, (and that I'll just touch on just briefly) advice has been given on other fora , for instant Thyroid UK (by Marz et al) - and that advice was very good (lovely thyroid folks) so I'll echo that briefly simply to put it in contex with your questions here
Individual absorption : Factors like age and digestive health can affect B12 absorption
It plays a central role in cellular detoxification, the neutralisation of reactive oxygen species, and the regeneration of other antioxidants including Vitamins C and E
B12 THERAPIES: INJECTIONS/SHOTS Vitamin B12 injections, also called shots, are one of the most common and effective ways to treat a deficiency
These links support internal navigation for compound comparison rather than consumer-style recommendation
Without intrinsic factor, oral supplements won't work, making injectable B12 for humans often necessary