Hello,
1. Currently not, but thanks for the suggestion. We'll think on that.
2. Adaxes custom attributes are stored on Adaxes backend, which is ADAM or AD LDS, and thus any limitations to such attributes are also imposed by ADAM/AD LDS. There is no hard limit as to how many values you can store in a multi-valued attribute, because the limit is applied not to the number of values, but rather to the size of the attribute (in byte). The practical limit is around 1200-1300 values. Thus, storing the information you request in an external DB or some sort of CSV / TXT files would be a better option.