The static text of the various messages and field names of the Feedback module (or any other module which is localizable) are edited using the Admin --> Languages tool. In the Local Resources branch of the language editors Resources tree drill down through the following nodes:
Local Resources > DesktopModules > Feedback > App_LocalResources. The texts that you mentioned are found in the file Feedback.ascx.resx.

Regarding redirection following submission to a page not included in the site menu, it is not possible to select such a page in the Feedback module settings. As a work around, you could temporarily include the page in the site menu, open the Feedback module settings, select this page for redirection and update the settings. Then, edit the page settings to un-check the Include in Menu checkbox. The disadvantage of this workaround will be that if you later edit some other Feedback setting and update the Feedback settings, the redirection page will revert back to <Not Specified> since the non-menued page is no longer available in the redirection page drop-down list.

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner