No! Don't worry. No new law or legislation passed by Congress can go into effect retroactively. That would be unconstitutional. As long as one files a siblings' petition prior to the F4 category being eliminated, that petition will be accepted and processed.
By "eliminating the backlog" what is meant is that the pending cases which were filed prior to the date of elimination of the F4 category will be processed faster and more efficiently, until there will be none left.
Absolutely you can! Make sure to have a valid national passport, in addition to a state issued photo ID if traveling by land, with, of course a copy of the receipt of your currently still pending extension application. If traveling by air, a valid state issued photo ID will suffice.
Before you listen to any other nonsense, do yourself the favor of downloading the instructions to Form I-130, from the USCIS website www.uscis.gov I think those simple to read instructions best describe the monetary and criminal penalties for those U.S. citizens who engage in marriage fraud, then decide for yourself if it's worth it.
As your mother's petitioner, the law requires that you file an Affidavit of Support on her behalf, regardless of your income or lack thereof. If your income were not to be sufficient, you must then find a co-sponsor to file another affidavit of support in addition to yours. If you want to add your husband's income to yours or it is joint income, you then need to have your husband complete the "contract between sponsor and household member" supplement to Form I-864. The instructions for all...