How to get an accurate time of arrival to the en-route alternate airport during flight planning?
This is very much like the information mandated for EDTO/ETOPS.
Computerised flight planning systems should have the ability to calculate the ETP, equal time  point (aka CP) based upon ERA/ERAs selected. If 2 ERA are required due to the length of flight, depending on the geographical disposition of the ERA you'd get an ETP for each pair, ie ETP1 for DEP Stn/ERA1, then ETP2 for ERA1/ERA2, then ETP3 for ERA2/DEST Stn.
The earliest ETA for ERA1 would be a diversion from ETP1 and the latest would be from ETP2 and you would need to subtract/add the 1 hr buffer.
And so on . . .