I would say you do want to get her eating creep - setting up a creep area is a good idea, hopefully the older lambs will start to eat the creep and she will copy them. If no lambs are eating the creep you will need to feed all, including the ewes, for a few days or a week or so, the lambs will then copy their mums and start to eat it. Once they know what it is they'll follow you to the creep area to get it.
If you only want to cake your orphan lamb, you will need to get her eating creep somehow. Could you bar her into a pen somewhere overnight with some cake? Once she knows about cake, you could cake her when she comes to you - replace the milk with cake as she gets the taste for it.
I would not recommend weaning by watering the milk down. This will make their tummies feel full and they will be less likely to eat grass and cake, whereas you want them to feel hungry and eat more grass and cake as you are reducing the milk. So another time, just reduce the amount of milk. Generally I do not stop milk until the lamb is eating at least half-a-pound of cake each day. Sometimes you need to reduce the milk to stimulate enough hunger to get them on eating cake, but I would not stop milk until they are eating cake at that level.
So yes, you have made a few mistakes - but don't beat yourself up. She sounds healthy enough, and that's the main thing. Feel her ribs and spine - if they are standing proud of the flesh then she needs more food. If she has a reasonable covering and doesn't feel like a poster for Biafra then she's doing just fine.
