Are women (married and unmarried) eligible to perform Ekadashi vrata?
Narada purana says that anyone can perform Ekadasi Vrat. They include Brahmanas, Kshatriyas, Vaishyas, Sudras and also women
I shall describe another Vrata well known in all the three worlds, which subdues all sins and which bestows all the desired benefits. O Brahmana, to those who perform this rite with devotion to Visnu, (whether they are) Brahmanas, or Ksatriyas, or Vaishyas or Sudras or women, it bestows salvation. It is very pleasing to Visnu. The holy vow called Ekddasi Vrata! (holy rite on the eleventh day) verily bestows everything that is desired by men. O Brahmana, it should be performed by all means, sinceit is very pleasing to Visnu.
Since women is mentioned explicitly, it can be inferred that there is no prohibition for either married or unmarried women. Anyone can observe this vrat.