I'm in the same boat. I'm not positive on the answer, but I believe the cost basis of zero that you're seeing for the cash received is actually correct.
If you look at the information forms shared with investors by ATT (https://investors.att.com/~/media/Files/A/ATT-IR/documents/stockholder-services/time-warner/TWX%20Sh...), you'll find a long legalese description that says:
"Your gain from this transaction (if any) will generally be the smaller of the following
amounts: (a) the amount by which the sum of cash and the fair market value of the AT&T
shares you received exceeds your tax basis in your Time Warner Inc. shares or (b) the amount of cash you received in exchange for your Time Warner Inc. stock (in each case
treating any cash received in lieu of a fractional AT&T share as if such fractional AT&T share
was actually received and then sold for cash as described below)."
I think the net result of the above for most people is that (b) applies and your gain is the full amount of cash paid, so you'll see a zero basis on your brokerage statement.
But you're being paid out cash--shouldn't that reduce your basis in your resulting ATT stock? The section that says:
"The tax basis in your newly acquired AT&T shares (including any fractional AT&T shares) will generally equal the tax basis in your Time Warner Inc. shares less the cash you receive as part of the exchange ($53.75 per Time Warner Inc. share and excluding cash received in lieu of a fractional AT&T share), plus the amount of any gain recognized from the transaction (excluding any gain recognized with respect to a fractional AT&T share)."
So, if (b) applied to you above and you're recognizing a gain on all the cash paid out, then this section has you first subtract out that cash and then add it back in. You end up received cash that is all gain, but maintaining the same basis in your ATT stock as you had in your Time Warner stock.
You mentioned some additional entries showing high basis and low gain--not sure how to explain that, but you might look at whether these represent payouts on fractional shares in ATT that would have resulted from the merger in your situation.
