How would one decide on the 12 month momentum calculation, choosing between either straight RS or price/(52wk HI + 52wk LO)?

Flip a coin.
Of course, straight RS is simpler and extremely easy to find. No computation, just look at the figures.
Actually the 52 week Hi & lo are also easy to find. You just have to load the data into a spreadsheet and do the calculations and ranking yourself.

I have all this stuff automated with bash scripts (running with Cygwin on Windows 10) so it was easy for me to clone the RS script into the HL-52 script.

I ran only the top 10 RS for over a year then branched out to the HL-52 after reading Jim's post #4412

I ran an older Jim screen for several years. It did great and I split half of it off to the RS screen in Jan 2025. Sadly, that screen did absolutely *nothing* in 2025 while the RS screen almost doubled.

Also what are your thoughts on Cash vs ST T-Bill ETF when not in?

I've been spoiled by Fidelity automatically sweeping your cash into SPAXX.

Here's the deal. Most of the timing OUT periods are only a couple of months long. The amount of interest you'll get in 1 or 2 months is trivial, so it doesn't matter if it is pure cash or bonds.
But *some* of the out periods are very long, so you would want to put the money into some interest-bearing asset.
But interest-bearing assets that have good yields are very much subject to capital loss if/when rates change. It's dumb to open yourself to a loss in your supposedly "safe, waiting for the bear to leave" money, so you should put in in very short-term income assets like a 1-3 month MMF or T-bill.

FWIW. The last OUT period with my current timing scheme was the one month of May 2025. I was using a different timing scheme then which said to stay in. (You would have missed out on a 12% gain. Bummer.)
The OUT period prior to that was 3/2022 until 3/2022, out for 12 months. (You avoided a -19% loss)

"The moving average timing strategy makes the majority of its money by avoiding large, sustained market downturns. To be able to avoid those downturns, it has to accept a large number of small losses associated with switches that prove to be unnecessary. Numerically, more than 75% of all of MMA’s trades turn out to be losing trades." - philosophicaleconomics.com: PHILOSOPHICAL ECONOMICS

But also, "A strategy that makes comparisons to a single prior level (i.e., momentum) is vulnerable to single-point anomalies in the data, whereas a strategy that makes comparison to an average of prior levels will smooth those anomalies out." - philosophicaleconomics.com: PHILOSOPHICAL ECONOMICS
Hmmm, I may have to look into that. Jim has mentioned that a number of times.