Hi Ch
The time markers will indeed be seen by the controller. However, use of the 'Demand Contract' function can provide the controller with the necessary confirmation that the next significant waypoint beyond the crew rest/ETOPS time marker is correct. The controller may then elect not to seek confirmation of the route in the FMC where it can be seen that the Lat/Lon of the time marker will not result in any deviation from the cleared route.
The standard ADS-C waypoint event contract is 1 report per significant waypoint. The 'R-LONGSM' 18 minute reporting interval is established automatically by the ATC system/Aircraft, where that facility exists, and is in addition to the significant waypoint reports.
The 30 minute Met contract is rarely used but can be selected by the controller. For example, if there is a CPDLC log-on issue and the R-LONGSM separation becomes unavailable, due to a lack of CPDLC, the controller can cancel the R-LONGSM (18 minute) contract and issue a Met contract (30 minute) so that any unintended use of reduced separation in the absence of a CPDLC connection can be avoided.
The only other contract we use at Shanwick is the emergency contract, which, depending upon the aircraft manufacturer, will downlink a report every 2 or 10 minutes. (I've not read GOLD I'm afraid so I can't comment on the 304 seconds periodic contract you mention, but at Shanwick we don't use such for establishing periodic contracts).
The passing, by HF or CPDLC, of a 3 minute revised estimate applies to all aircraft, whether ADS-C equipped or not. Unless there is a credibility/conformance issue, significant and intermediate waypoint position reports do not refer to the controller but simply update the flight's profile in the ATC system.
However, by bringing to the attention of the controller the possibility of arriving over a significant waypoint >=3 minutes will enable the controller to intervene, (apply a time restriction and/or adjust speed), at the earliest opportunity to ensure that separation at the next significant waypoint will not be lost.
Why New York do not require the passing of >=3 minute revisions I do not know I'm afraid. Hopefully someone here may be able to inform us.
Hope the above helps.
RK