RA Down link will show that the system has issued an RA at one point of time but not that the pilot is following it.
The downlink itself may not show if the pilot is following an RA, but the altitude readout on the ATC screen will be able to confirm it within a few seconds -- or earlier if the downlink is delayed.
As for switching to TA with an engine failure, Eurocontrol ACAS Bulletin 9 (Jul 07) addresses it specifically:
TCAS II provides maximum benefits when operated in RA mode and when all RAs are followed promptly and accurately. However, when the aircraft performance is degraded (e.g. an engine failure, an emergency descent, etc.), it might not be possible to comply with a Climb”RA. Therefore,airlines must define clear procedures to address degraded aircraft performance situations, having in mind that:
•When operated in RA mode, non-compliance with an RA will adversely affect the efficiency of the coordinated RA triggered on-board the other aircraft.
•When operated in TA-only mode, collision avoidance is still maintained by the TCAS II of the other aircraft
The use of the TA-only mode is described in the following note that will be included in the next version of the ICAO PANS-OPS, Doc 8168, due in November 2007: “Note 2. -The normal operating mode of ACAS is TA/RA. The TA-only mode of operation is used in certain aircraft performance limiting conditions caused by in-flight failures or as otherwise promulgated by the appropriate authority.”
And for those who doubt whether a TCAS RA must be followed when there are contravening ATC instructions, the same Eurocontrol bulletin makes it very clear that following an RA IS MANDATORY despite such ATC instructions:
Notwithstanding ATC clearances, TCAS II triggers an RA when it predicts that a risk of collision exists if the aircraft continue on the same trajectories. Therefore, although it might appear as a nuisance in hindsight, an RA is always necessary at the time when it is generated.
...as well as in ACAS Bulletin 8 (Jun 06):
Prompt and accurate pilot response to all RAs is key to achieve maximum safety benefits in all airspace, including European RVSM airspace.
This will also minimise the risk of a domino effect with a third aircraft at an adjacent RVSM flight level.