Personally, I would NOT want my daughter bringing men to my house to stay with her in her room. Yes, I know teens and college kids often have sex, but to let them do it under my roof and have him stay in the bedroom to me, is sort of like enabling/encouraging it, not to mention, it is disrespectful to my household and I honestly don't care to see a parade of men coming through my house every few months, because at that age, it is very rare for a relationship to be serious and long-term, to the point they never date or sleep with anyone again. Again, this is MY personal viewpoint on the subject, you're free to do as you please, as are other moms who may not agree with me.
You may call me old-fashioned I guess, but many of the guys I'd met or date in college would often tell me that neither they nor their brothers were allowed to have a girlfriend sleep over, and one even said his brother got kicked out of the home when his parents found a used condom on their bed and realized he had had sex with his girlfriend when they were out for the day, so maybe not wanting sex to happen under your roof is not that uncommon. A spare room or couch would be more acceptable and in my opinion, a good compromise in helping the young man save up and not spend on hotels, without them overstepping boundaries. Regardless of his accommodations, I hope you discussed the possibility of STDs and the use of a condom, as there is no guarantee that either of them is monogamous.
As to whether she is playing the guys or not, that is not your business. You have no input or right to interfere. Let her and the guys figure it out. Cheating and having your heart broken is always a risk in any romantic relationship. Yes, perhaps she likes the guy in college but he's not giving her the attention or love she seeks so she's keeping him in the back-burner and in the meantime, seeing the Scottish guy. If he comes around and does start liking her, she may choose him over the Scottish guy, but that is a risk that is especially common in a long-distance relationship. I am sure that the Scottish guy has considered that scenario (and who is to know if he is in a similar situation, waiting until something local comes around), and if he still wants to spend the time and money, that's his choice. Why do you feel you owe these guys something and warn them about your daughter not being serious? Her relationships, her business.