That's really pressurising of them all. They should leave you alone to enjoy your night out. TBH if they're going to behave like this you should put your phone on silent so you're actually giving your attention to the person you're with, not constantly replying to texts and taking calls from people who aren't there, inevitably ignoring her for a few minutes each time while you do so. It's rude.
I don't know why you keep suggesting that you are happy to prioritize her when it's blindingly obvious that you prefer to take part in the couple dates when those are available and that those are your top priority. So you DO want to keep your options open then by not booking things with your single friend. You can deny it all you want, it's just not the truth, you contradict yourself all the time with the truth which is that you do want to keep your options open and not have such advance plans.
I don't know the single friend's work patterns but it could be that she has to organise herself quite far in advance or else run the risk of not being able to get time off/get roped into extra shifts because she's available and others aren't.
She's possibly also having to be so proactive in her friendships because she's finding herself continually being bumped from people's priorities when they get offers of social events which they can attend with their DP. Most people prioritize socialising with their partners or spending time as nuclear family, not going out much. That's fair enough, everyone is free to do what they like. It does leave singles being ignored a lot of the time though once people get into their thirties.
I expect she's trying to counteract that by making advance plans, so she doesn't turn round one day and find all her friendships have faded and she's got no social life. That's not your problem of course but you need to be honest with her about not wanting to make advance plans. The unspoken part of that sentence is always because I want to keep my options open. Whether that's options to work, to stay home doing nothing, to be with family/DP or to go out with someone else. You're right, it's not nice having to say that to someone, nobody wants to be effectively told they're not a top priority, but it means she knows where she stands and she's free to then decide to prioritize other friendships if she prefers.
What you're wanting is not to have to let her know she's not the top priority but still have her thinking the sun shines out of your arse/she's your best friend/however you want to word it. That's unrealistic, you can't have both.