Copts do not allow divorce - is their position basically the same as the RC, up to and including widows/widowers to remarry? Or are there significant differences in how Copts understand this prohibition.
Divorce is allowed (but not encouraged) in the case of adultery. The same act that consummates a marriage can break it. Divorce (or more properly annulment for these cases) is also allowed in the case that the marriage cannot be consummated, or in the case that there was serious deception from one party going into the marriage.
Death does not end marriage. However, for the sake of salvation in our weakness, a widow or widower is allowed to remarry. The ideal though is to remain faithful to the departed spouse, which is why there is no crowning at a second marriage. It is a penitential rite because it is a sin, but a lesser sin than fornication (keep in mind that sin doesn't mean doing something illegal, but falling short of perfection, missing the mark). A member of the minor orders (chanters, doorkeepers, readers) who remarries once is not allowed to advance, and one who marries a third time is no longer allowed to serve. Of course, members of the major orders (including subdeacon) are not allowed to continue serving if they remarry.
Subdeacons are a Major order in the Coptic church?
People seem to disagree about this. The 7 minor orders are listed in the Liturgy as doorkeeper, chanter, reader, exorcist, monk, nun, widow. Subdeacon is not listed with them, but is mentioned with the deacons. People teach that subdeacons are among the major orders. I know that this contradicts every other tradition. I guess there's nothing wrong with having this difference if it is correct: in other traditions a minor order that assists the deacons could be called a subdeacon, and in the Coptic tradition the deaconate could be structured as subdeacon, deacon, archdeacon: in one case it would be a minor order below the deacon, in the other it would be a lowest rung of deacon. But it's also possible it got confused at some point and got put on the wrong list. Who knows? In practice, it seems to straddle the major and minor orders in terms of respect, duties, restrictions, etc., in all traditions.
Of course today, Copts teach that there are many types of deacons: chanters, readers, subdeacons, deacons, and archdeacons. Of course, this is clearly a confusion, with the minor orders of chanters and readers, which are not levels of the deaconate, but are their own roles for laymen, having been confused as levels of deacons since deacons are now extinct and these minor orders fill their roll. Since all the chanters and readers think they're deacons, you'd be hard pressed to find someone who can explain exactly where subdeacon fits in the Coptic tradition.