I don't think I've ever seen women in the choir whenever I've visited Coptic churches.
Are women allowed to sing in the choir in Coptic churches? Or have I just been to parishes that happen to not have women interested in being in the choir?
A somewhat complicated answer. Ultimately, it is indeed allowed for women to sing in the choir. However, in Egypt alone, this never happens. Coptic people are too influenced by Islamic chauvinism to see women stand with the rest of the "Singers" to praise God, and this later on lead to a misunderstanding of what the Church teaches (i.e. Coptic people themselves are not chauvinistic in this regard, they just treat this issue like they would treat the issue of reading the gospel, going in the altar, teaching, or being a priest...they feel the Church disallows this, which is not true), and this idea carried on to the US. In the US, priests usually won't try to deal with the situation because they know how this would lead to an "outrage" by Coptic people. I've met several women that outshine any subdeacon in the Church hymns, and yet even they seem to have a belief that women should not stand in the choir.
At the same time, many parishes do have women who are not interested in being in the choir or evev learning the hymns anyway, so the feeling is mutual. I don't know whether to blame the women for being less interested or for the Church in general for not sparking interest to women. In the diocese of California, the bishop there started to consecrate women to be in the choir and to even lead in Church hymns. As predicted, this did cause outrage among Coptic people. I know a female friend of mine from California who is also against this, and she loves the Church hymns, who even goes on to say that her priest won't allow it, which I find it hard to believe. I just think her church is filled with ignorant folks to which the priest is not taking any drastic steps to cause any outrage.
Another part of the complicated answer is that the Coptic Church historically really was never structured to have a choir. It was structured to have the congregation involved. However, as the congregation seemed to be less interested in learning all the Church hymns, a choir evolved, or at least one man who lead the congregation, called a "mu'allim" (the master, or the teacher) developed in our Church. So it never crossed anyone's mind to include women in the choir since then probably (although convents have no choice but to include singing women in their all-female choir).
So this is generally my experience with Coptic parishes in the states.
And to answer someone else's question, yes, Coptic parishes generally don't kick out non-Orthodox people after Liturgy of the Word. It just so happens that monastics like to keep the tradition of old.