Please listen to Shannon, below. When your girl calls, continue to go as you have been doing. You're going to get some "Let her cry it out" and "She's manipulating you" responses on here, I'm sure, but I don't go for that. Go, and be sure you go silently -- don't engage, talk, soothe, certainly do not scold or fuss in the middle of the night. Cover, hug, go, repeat.
This will pass.
You are frustrated with the idea that she was a great all-night sleeper and seems to have backslid. Can you step back a bit and think of her not just as waking you but as a kid who is entering a stage that's pretty normal?
You didn't expect this stage because the secret that all the "get your kid to sleep all night long" books and programs (and most parents) don't tell you is that children's sleep patterns change ALL through their childhoods and yes, into the teen years. So having a perfect sleep-all-night baby and toddler can indeed turn into two- or three- or four-year-old who is wakeful and wanting you. It is normal and it is not a product of her trying to manipulate you. She's not old enough for that just yet.
She may be undergoing a growth spurt; you've probably heard that that can make kids sleep more, and it does do that to many kids, but it also can make a kid more restless. Or she may be starting to have nightmares. Two is an age when a lot of kids do. But if you ask her if she had a bad dream, she might say "No" because she doesn't even really understand yet what a dream is, or can't recall it; she only wakes up scared and knows only the fearful feeling -- not what caused it. So she calls.
Here's what our pediatrician (mom to three) told me long ago and I agree: Refusing to go to a kid this age can make her MORE insecure, not less insecure. It does not teach a child this young "You can get over it on your own" if you and/or dad don't go to her. Instead, her young mind will interpret it as "When I'm scared/sad/alone, they don't come." That breeds the feeling that she can't depend on the adults to respond to her. She has no concept of keeping someone else awake; she probably doesn't even really realize you adults ever sleep -- she only knows "I am scared/wakeful right now" and she doesn't want to be alone. She is not capable yet of thinking, "I shouldn't call, I will wake up mommy and she needs to sleep."
By going to her now and for a while to come, you will lay the groundwork for her being able to soothe herself better later on. Meanwhile, yes, it's tiring, but it isn't spoiling her; there is some internal reason she's waking this much and wanting you, and -- this is important -- if she is happy and not clingy or demanding during the day, this is truly a product of nighttime waking and possibly dreams. Go to her. Again, don't talk, or engage. If she gets up to come to your room, get up, put one hand on her shoulder and walk her silently back to her bed, cover her, hug and go. Every time -- no talking or engagement (so she doesn't start to think it's a game) but a return to bed. It will pass though it feels eternal when you're in the throes of it and you may be for some months to come.
As for sending in daddy, does he do the bedtime routine, do you take turns, or do you do the bedtime routine? If he's not part of the routine, or does it very seldom, start trading off so he is putting her to bed at least half the time if not a little more. It can help if you're out of the house at that time so she can't run to you. It gets her used to associating him with getting to bed and might make it easier to for him to go to her in the night instead of you. It won't be a quick fix, though.
I'd stop trying the changes to bedding, incentives, etc. She does not associate those things with waking in the night and wanting you -- when she wakes she is totally in that moment of time and not connecting it with any incentives you promised earlier. She is too young to wake in the dark and stop herself to think, "Wait, if I don't call out, mommy will give me X in the morning." She can't grasp that kind of far-off reward yet at all. And to her at that second, morning is a lifetime away, and so is last night's bedtime and any discussion of incentives.