After escorting her, she agrees to meet up in an Old Mill, after that of course I did some quest now I tried meditating until Dusk and Midnight and still couldn't find her, is there still a chance to see her in the Old Mill?

She should be waiting in front of the door of the mill. She will only be there in the evening, but I am not sure at what time exactly. I suggest to try 7-8pm and if she is not there, to rest in 1 hour steps till she appears.

Don't forget to bring some wine ;)