I definitely think you need to get clarification if you are able to bring a plus 1 to the resort at all. If yes, then they should definitely be invited to all wedding events, otherwise that seems very odd.
We got married in July and added plus 1's to the guests who didn't have a significant other but kept it to the specific guest for some older adults that were family friends and wouldn't be bringing anyone along anyway.
We felt that if they were willing to spend the amount of money it costs to attend our wedding, they should be able to enjoy their 'vacation' with anyone they want. We also didn't invite a ton of people to the wedding knowing we were having an at home reception a month later so we had 40 guests with us. With the wedding packages the hotel offers, they include a specified amount of guests so we only paid for 10 extra people and we could hardly complain about paying an extra $25 for 10 people when at home it would have been at least $100 per person per plate. And yes we paid for each chair at the reception and the ceremony.. but again, these guests spent money, time away from their kids, jobs etc...to be with us, I would have been happy to pay for an extra chair, meal, excursion that would have allowed our invited guests to enjoy their time with us and with their plus 1 weather it be a friend of theirs or a new relationship.
In the end we only had 2 people come as singles due to either no significant other and it being their choice to not bring anyone and another person's wife was out of the country for work. My husbands sister, a bridesmaid of mine had a bad breakup a month prior to the wedding and she ended up bringing her best friend which we didn't mind at all. If she was willing to get all of the hotel info changed over to someone else and her friend was wiling to drop everything a month before the wedding and be okay taking a last minute vacation, why not?
I agree that a destination wedding is more intimate but if someone brings along a person that you aren't that close with, you're hardly going to notice because you'll be too wrapped up in the excitement of the wedding, planned out events, your soon to be hubby/wife and enjoying everyone's company. And who knows, you might come to be great friends with this more random plus 1!