Trig Mode S is 1090. Is the other frequency, 978, not US? The Skyport document mentions "aircraft broadcasting electronic conspicuity with SIL=0" . Would this include Mode S without ADS-B?
"TDAs will not be activated for 24-hour periods or 24-hour periods for several days at a time.
Skyports intends to use the TDAs only when required and will deactivate TDAs outside of notified hours."
The surgeries on Coll and Iona operate for only one hour, Monday to Friday. I suspect there is no permanent doctor on either island, and that they travel from Tiree and Mull respectively for the surgeries.
They likely also may visit, out of hours, for emergencies.