TR.DealDate/TR.DealStatus/TR.DealTarget etc can indeed be used to quickly retrieve very basic high level info on the most recent deal involving a company. Btw I don't reproduce the issue you reported originally. On my end in DIB for REN I see TR.DealDate returning 11/19/2018.
The dataset I referred to in my previous response is a trove of additional info on M&A deals containing over 1600 data items of which about a 1000 can be used as criteria in deals screener.
It should be possible to compile the list of US stocks that are not target of an active acquisition, though it likely would require multiple steps. The deals screener returns deals, not companies. Setting in the deal screener the criteria of deal status equivalent to "deal currently in progress" and target is a public company headquartered in the US results in more than 15K rows of data, which is the max deals screener can return. To compile the list of US stocks that are being targeted you'll probably need to narrow the screener criteria and run multiple screens to cover your entire universe. Then from the retrieved list of deals you can compile the list of unique targets, exclude this list from the universe of all US stocks and that will be the end result you're looking for. This said I'm not a specialist on M&A data. Maybe there's a better way. In general if you need help with constructing a screen, the best resource is Refinitiv Helpdesk, which you can reach by either calling the Helpdesk number in your country or by using Contact Us capability in your Eikon application.

The key to query deals data is the Deal ID, not any company identifier. Since you wouldn't know the Deal ID, the way to retrieve deals data is through the screener. To explore the screener click on the Screener button under Thomson Reuters tab in Excel ribbon. The screener defaults to the universe of public companies. You want to change the universe to Deals and then follow the wizard to construct the screener expression and insert it into Excel worksheet. Then you can copy & paste the screener expression into your Python script. E.g. the call below returns all deals where Resolute Energy Corp is the target or the acquiror. The identifier used for Resolute Energy Corp is its PermID 4297165658.

ek.get_data("SCREEN(U(IN(DEALS)),IN(TR.MnAParticipant(DealPartRole=T,A),4297165658))",["TR.MnAAnnDate","TR.MnATarget","TR.MnATargetPermId","TR.MnAAcquiror","TR.MnAAcquirorPermId"])

The data items for deals data can be selected by customizing the columns in the screener wizard. Or they're available in the Formula Builder in Excel or in DIB. In DIB they're under Content Classification: Deals and they all start with TR.MnA. You will notice that the above request returns Deal ID numbers in the Instrument column. These Deal IDs with "@dealid" appended to them can be used as keys to retrieve TR.MnA* fields. E.g. the specific deal you mentioned has Deal ID 154085238344, which can be used as follows:

ek.get_data('154085238344@dealid',['TR.MnATargetMidInd','TR.MnAAcquirorMidInd'])