The fiscal sponsor must be a 501(c)(3) organization as recognized by the IRS. The fiscal sponsor cannot be an organization whose tax-exempt status is pending. The sponsored project must be able to qualify for exemption under section 501(c)(3), even if the project has no intention of ever applying for it.
Pressing the Ignore button on the listed invitation on the LinkedIn site is effectively a soft rejection. The message then moves to your Archived folder. You can later choose to open the message again and accept the request.
CONTACT THEM AND ASK THEM. IT WOULD DEPEND ON WHY YOU THINK THEY WOULD OWE YOU BACK PAY. FOR EXAMPLE, IF THEY KEPT DENYING YOUR CLAIM LIKE THEY USUALLY DO, THEN YES THEY WILL HAVE TO PAY YOU FROM THE START OF YOUR APPLICATION.