As far as answering your question about whether a marriage cert would show whether a father was deceased, the answer is that it sometimes does give that information, but not always.
However in this case you know from the will that he was dead by 1897 and that he was the father of a child born 1870. Therefore it follows he died between 1870 and 1897, does it not?
There are 2 possible deaths in that period, in Co Antrim, on the LDS Beta site:
Robert Townsley born 1803, death regd April – June 1883, Belfast, Volume 1, page 266.
Robert Townsley born 1825, death regd Larne Jan – Mar 1894, Volume 1, page 543.
I’d say the Larne one was the more likely of the two.