It has been my experience that normal amounts of dark malts in typically alkaline waters do not produce low mash pH. Other brewers who actually measure mash pH confirm this. But I can't disallow the possibility that someone, somewhere is going to get a mash pH which is too low in a dark beer. So I have changed my rule: never add chalk or bicarbonate to brewing water or mash to "Never add chalk to water nor to mash unless a mash pH measurement with a properly calibrated pH meter indicates that it is necessary."
Lactic acid is something we try to avoid (i.e. by preventing contamination with lactobacilli) in general but sometimes we want it. Examples: sour creme, creme frais, sauerkraut, kimshe, sourdough bread/pizza, Berliner Weiße, lambic and the sauermalz/sauergut that it is used in most lager brewing in Europe for mash pH control. Lactic acid actually tastes pretty good (IMO) but when it's out of place it's out of place.
Citric acid used to be a staple of home brewing. Look back at some of the early homebrewing books that came out of the UK. Almost all the recipes called for some. I find it interesting that in those days it was recognized that acid was needed and it is only now that a few vox clamantis types are beginning to get this notion back in front of homebrewers. So yes, citric acid should work. The reason I think it's not used more is because it tastes, well, citrusy. I'll leave out unkind remarks about the beers some of you American hops lovers produce.