In general citric and ascorbic acid will not have a significant impact on microbiological stability in terms of spoilage.
Ascorbic acid will help reduce the rate at which oxidation occurs, and can protect against colour change/darkening not only during shelf life but also during pasteurisation stages, where the heat used would otherwise significantly increase the rate of oxidation reactions.
Citric acid can be helpful for managing pH levels, particularly in products that are naturally lower in acidity (vegetable juices, banana etc), and this has
beneficial effects in terms of pathogen control, but it won't generally significantly help with yeast/mould - lemon juice concentrates with natural citric acid contents of 20+% will still quite happily ferment, given the opportunity
I don't know if it's significantly different in the US, but we don't see much canned juice around here these days so I'm a bit out of the loop on it, but I have seen canners successfully use ultrafiltration as a microbiological reduction step, although this only works with clarified juices (and suitable hygiene controls at filling) - anything at all cloudy will just clog the filters up.
Hot fill might also be an option.
What's the usage/labelling position on Velcorin / Dimethyldicarbonate (DMDC) in the USA? I have seen one or two instances of this being used and not declared on the label, on the basis that it isn't present in the final product, but I don't know if it's permitted and/or declarable in the FDA's view.