I don't think I would want Tolkien to get graphic about it and it is possible that Tolkien did not want to be specific for whatever reason.
But to me it depends what view of middle-earth you take. To me it is an alternate universe where all higher beings live out the Catholic ideal (no extra-marital or recreational sexual activity)
, and the Armies ofthe west was probably the only ever not ministered to by practioners of the oldest profession. It contained many of the nasty truths of our own world both past and present but they were contained to the lower orders (cf Brodda the Easterling), this seems to me more likely than assuming that there was lots of sex going on that Tolkien studiously ignored, and that "the truth"was closer to the fanfic... which would make Tolkien a "historian" on much the same level as the chap who wrote "The agony and the ecstasy". Equally unlikely to me is the idea that orcs would draw the line, that they would have a moral code that would prevent them committing rape if they were capable of it when they kill without thought. Historically, rape has played a part in military conflicts and in certain circumstances still does (former Yugoslavia, parts of Africa now..). In these instances rape is about power as much sex and I while I have never subscribed to the glib theory that all men are potential rapists - I am sure that war may make rapists of men who would never have done such a thing in their normal lives.
So, on balance of probabilities , I think that Celebrian was raped - unless there was a plan to hold her to ransome which would explain why she was not killed immediately. If she were not raped it is hard to imagine what torment would be so bad that she could not recover despite Elrond's best efforts - OK merely being the prisoner of orcs would not be fun but Celebrian is in the unique position of knowing that it may be goodbye for ever to her children if she goes....
I think we have to remember that rape was a taboo subject until recently - maybe even since the prof's death,and was long regarded as a fate worse than death and victims were stigmatised. This lingers on .... I think it is quite possible that the prof left indications for those eyes that can see them but itisequally possible he sketched out the story only briefly and never chose to investigate further....