I'm not 100% certain about how insurance companies calculate a policy quote using information in your credit report. My bankruptcy (I've had two) has not negatively impacted my insurance (home or auto) in the least, then again my credit history is in pretty good shape other than the bankruptcy. Your experience is that some negatives in your credit history are going to be a problem in getting a competitive policy quote.
There are some points you made that trouble you and trouble me, namely confidentiality.
I am uncertain how far an insurance company is going to take your mother-in-law's request. She will be requesting a quote for coverage that is not for her and this is NOT to add you to her policy -- she wants to find you a new policy. Secondly I am not sure how far an insurance company is going to go with her producing a Social Security Number that does not belong to her. The problem with Identity Fraud today has changed how businesses handle this confidential information. Now if she is looking to add you and your husband to her existing policy that may be different. But that is not what I understand she plans to do. She plans to take your information and shop for you. I can't imagine a legitimate company would accept her signature as a waiver to use Social Security numbers that are not hers. Nor can I imagine a legitimate business would accept her authority as the legal approval to pull your credit file. So I think her good intentions may be thwarted because the social security number and credit history is not hers. If this is the case then you can relax and emotionally exhale.
I also think changing your legal address to her address to obtain confidential information is unacceptable. Her intentions are honorable, but philosophically I believe she needs to honor your right to privacy and respect your boundaries. You are a married woman and even though she is older she needs to view you and your husband as an autonomous unit that is separate and distinct from her.
If she wishes to write checks to pay for your insurance premiums that is extremely generous. However, it does not give her the right to delve into your private finances and access confidential information.
And I am rather certain insurance companies see it that way too. In viewing this from an insurance company's perspective, they would have a huge liability if you were to go back to them over pulling and releasing information to someone you have not authorized in writing.
I'm hoping your mother-in-law will be turned away and she will call you to let you know she couldn't proceed with a quote without you. If that's the case you can be gracious and thank her; let her know you are or will be getting quotes from other companies and you'll keep her posted.
Hope others can add insight.
Carolyn