Is it permissible to add the letter "waw" when saying "wa ashhadu anna Muhammadan rasulullah" (and I bear witness that Muhammad is the Messenger of Allah) in the Iqamah, and does that affect the validity of the Iqamah?

The wordings of the Iqamah are the same as the wordings of the Adhan, with the addition of "Qad Qamat Al-Salah" (the prayer has indeed been established) after "Hayya 'ala Al-Falah" (hasten to success). This is by agreement of the Madhhabs (schools of thought). The wordings of the Adhan and Iqamah are devoid of the conjunction "waw" (and) between their phrases. Whoever inserts "waw" between the two Shahadas (declarations of faith) has deviated from the Sunnah, but this does not invalidate the Adhan or Iqamah, as the meaning is not changed by this addition.