In my personal experience having dealt with some individuals who went through a giyur lechumra, it has depended on why they were going through the giyur.
If as far as we know, there was nothing wrong with their (or their maternal ancestors) conversion, but we simply don't recognize (some or all of) the members of the Beis Din through which they converted, they are counted for a minyan. (I never had experience with the eidim aspect, but my hunch is we wouldn't use them for eidim in any situation in which we are choosing eidim, and if they were the only available witnesses to something that happened, we would follow the rules of regular safek, and for example would not extract money on account of their testimony.)
If we have a specific known issue with their status, such as we have reason to believe that their mother did not accept mitzvos, or one of the members of the Beis Din is known to publicly desecrate the Shabbos, just that we don't know if 15 years ago when the conversion happened if he was or not, then they are not counted for a minyan (and I imagine certainly would not count as valid eidim).
I don't have any specific sources for this, but this has been what I've seen practiced.