1911 marriage – the only real way to know who married who is to buy the marriage certificate, which you can do on line from the General Registry Office. To avoid wasting money on a certificate you can (a) specify both parties names, so in this case that would Harriet Williams and Thomas Dixon, and (b) don’t provide the GRO reference (Vol 6A Page 292), just give them the year, and (c) as you know Harriet’s father’s name from the census, specify that as well. Then if there is not a match, your payment is instantly refunded. This is a link to the GRO web page:http://www.gro.gov.uk/gro/content/certificates/default.asp
It is possible that Harriet Jane and Robert had children, but not that likely as she would have been 45-46 at the time of marriage in 1939.
I have absolutely no idea about Robert’s birth details – the register of marriages does not specify ages or birth locations of anything other than names.
Gaynor Williams was 65 when she died (the death register gives her age). The 1911 census shows a “Gayney” as a sister to Harriet. My guess is that the probate entry you found is her.