Is my marriage religiously valid despite having no guardian (wali) upon my conversion to Islam, and with the suitor's friend acting as both my guardian and a witness at the same time?

The fundamental principle for a woman who has no wali (guardian) is that her marriage contract should be solemnized by a Sharia judge or a marriage officiant (ma’dhoun al-ankihah). If the woman is in a place where there is no Sharia judge, it is permissible for her to appoint a just Muslim man to solemnize her marriage. In the presence of a ruler (sultan) in the country, scholars have differed on the permissibility of her appointing a trusted Muslim, and the relied-upon opinion is that it is not permissible except in cases of extreme necessity.

Based on this, the aforementioned marriage contract is valid as long as it was done with the permission of the judge or the marriage officiant. There is no issue with the same man solemnizing the marriage contract and testifying to it, as everyone present at the contract from among the men is a witness, whether he signs the contract or not. A group of scholars have validated a marriage if it was publicized and announced among people, even if two men did not witness it.