However should I be stupid enough as follow the word of the law I could not enter into air to air comms to advise the aircraft that I have relayed the distress message for or initiated one on his behalf nor advise him his wing is, indeed, actually on fire or that there is a suitable airfield in the vicinity of which he may not be aware.
You may do all of that. If the 'mayday' aircraft is able to receive messages then they will, of course, hear you relaying. The ground station will be in the best position to co-ordinate actions that should follow. If the aircraft is able to divert then it will not be a mayday situation but a 'pan' which is classed as an urgency and ATC will best advise of a suitable aerodrome taking into consideration a number of factors; it would follow that you would then relay on behalf the ATC unit to the aircraft in question. Cluttering the frequency with unnecessary calls will not help, even if well meaning.