How do you harvest the seeds

Cut and Paste: The little spiny ovary of the female flower develops into a fruit or seed capsule about the size of a golf ball after pollination. The seed pods may be green, pink, or red (depending on the variety), but gradually age to brown. Each spherical, seed capsule is thickly covered with soft flexible spines and has three sections that separate when the seeds are mature. Each section contains one seed that is ejected, often with considerable force, when the carpel splits open. It's for that reason I never bother with the seeds of this variety. Easier just to buy more.

Is this the type of castor plant that I can use to make castor oil?

Yes. Castor oil is made from the seeds, so you would really need to have very many plants. If you search "how to make castor oil" on youtube a bazillian videos come up.

Does anyone grow these to repel mosquitos?

Yes. Whether or not it works I cannot say.

Can they grow to 9ft?

DEPENDS ON SOIL QUALITY, AND THE PLANTING TIME. I HAVE SOME AS LARGE AS 8 BEFORE AND ABOUT 6 FT DIAMETER. I LIVE IN ARKANSAS, PLANTED VERY EARLY, AND MINE ARE ABOUT 2 FT IN HEIGHT CURRENTLY.