Might be a boy, how old is it? With BR it's hard to tell because the females can sometimes develop large combs and the males can have small ones and of course the feather markings are the same, soooo....... But I've read that females can take on male traits like crowing, protecting, trying to mount & mate and can even develop spurs, so once again????? However, if it lays an egg, then there won't be any doubt, well I take that back, that's not true either, some say that if a roo isn't present that the alpha female can take on the male roll as I mentioned above and then even stop laying eggs, so therefore, maybe they'll never even ever start depending on the age and flock dynamics and then you'll never know. LOL!!!
Not true. Barred rock feathering is different in males and females which makes them one of the easier breeds to sex. Males will have more white in their barring and females will have darker barring.
How old? And doe it have the skinny spiky feathers coming out of its saddle too? this one appears to have some streamer type hackle feathers, so I would want to see the saddle and know how old before I would say. At first I thought pullet, but then thought twice with the hackles.
The others are just as you said, I would say. Red Sex Links are most times lighter than that red one you have, but it doesn't mean it is not a sex link. If it is a RIR, either way she will lay a brown egg.