You need to report what is shown on your form 1099-B even if "non-covered".  Non-covered means that the proceeds are reported to the IRS, but the cost basis is not.  So you will enter both the proceeds and the cost basis, and the difference will be a taxable gain or loss.

Does that apply to stocks exchanged during a Merger & Acquisition?

Askking because according to my tax pro at H&R Block, they are not.

If you received a form 1099-B, you need to report the cost basis and sale amount that applies to it on your tax return. The IRS will receive a copy of it and look to find the entries on your tax return. If there are sale proceeds listed on the form 1099-B but no cost basis, the IRS will assume the sale amount is a taxable gain and you will receive an assessment sometime down the road for the taxes applicable to it.