Like Jim says, she's just in a different place in the pecking order. I've had several hens over the years try to become the "rooster" when there are no actual roosters around. This can take many forms although I've never had one crow as excessively as your hen seems to be doing. Since the former dominant birds are not there, she is taking their place, although for her, she's not only acting as the sentinel, she's actually calling to the other area
chickens and filling the roosters spot. Now, you might be asking why your other hens didn't do it, but dominant birds don't always do everything that a rooster would do. Crowing is the least common, usually they just stand watch more, eat less, stop laying eggs, lead the group to new grazing areas, and mount other chickens. In my
experience crowing is the rarest behavior for a hen.
I think you are accurate in your observation that this behavior coincides with the death of the other two hens. She's the boss now and she is trying to fill all the roles of the rooster. Unless you get a rooster, which you can't, she will probably continue to act this way unless another hen becomes dominant. As the chicks get older, some of them will test her and if they "win" (usually this is just mock fighting), she will stop behaving this way.
The chances that she is a hermaphrodite is very rare. More likely she has just a bit more testosterone (even hens make a certain amount of it) than the average hen and the sudden change in social climate has her thinking she needs to be the boss and act like a rooster.
Nature abhors a vacuum and chickens seem to need leaders.