I keep saying to myself that a tiny baby can't be manipulative because I don't want to regard DD like that, and that wants and needs are the same thing for the first year of life, but does that really apply to an 8 month old that acts like a 12 month old?
Yeah, we must have heard that same 'wants/needs' story somewhere -
but you've asked the million dollar question.
In hindsight, I would suggest you treat her like her mental age (which could be more like 18 months - visit a daycare center if you get curious)when she's in a good mood and like her bio age when she's tired, cranky or sick.
BTW - it's cool that you are using signs with her...I think that will be a big help. At my son's daycare one of the mom's taught her daughter the sign for 'more' and lots of the other families copied. It was so interesting to me how my son 'got' that more could mean 'more cookies' 'more kisses' 'more uppies'... whatever.
So maybe we can think 'manipulative' in the Montessori sense of a tool that can be used in Math, for example. I don't mind being a tool for my child - although it changes what's appropriate to be a tool of as they age. One of the other moms used to say to my son, "Oh, that's great problem solving!" sincerely, as she would pry the scissors out of his hands, for example, when he used his noggin to get into some minor trouble that I would just as soon wished he hadn't been so interested in, such as trying to work the car door lock without a key.
Not the faint of heart or shy of manner...
L&ML,
Grinity