There are no recoords of Walter Oldfield Lowe (or variant spellings) arriving in Argentina by boat on the CEMLA site or the Irish Immigrant ships site (
https://www.irlandeses.org/paxlist20.htm). However, their data may be incomplete.
As you say, there is no death for any female surname Lowe around the dates expected on the agrgibrit or Family Search (Catholic records) or any other site.
Walter was widowed on 1895 Argentine Census and school admission records for Sefton Park School Liverpool have sons Wm Lowe (walter William) as being born on b. 17 Jan 1889 & Robert Lowe's as 1 Nov 1890.
Their mother's death then somewhere between 1 Nov 1890 and the 1895 census. There are no children with him on the 1895 census with Walter, suggesting they could have been sent back to England, since they are not showing on separate searches.
One of the pages on the Argibrit site is not linked to anything (the burials at the Dissenters cemetery) and there are only a percentage of images of actual graves on Findagrave for this and other surrounding cemeteries. Graves in Argentina are at any rate usually re-used and remains removed after a relatively short period of time, so there would unlikley be a gravestone and so no record would appear on FAG. It might be worth contacting the cemetery to ask if they have a record of a burial, with a surname Lowe or ending in Lowe, since their maiden name may be also before the married name. However the records may well only be at the Methodist archive, which is what Jeremy Howat used to create his databases. Don't let the language put you off, Google translate will work probably be fine.
Cemetery:
https://www.cdisidentes.com.ar/contact/Methodist Archives:
https://iglesiametodista.org.ar/registros-historicos-de-las-comunidades/This site has an email contact (if it still works) for general archive covering Rosario
http://censoarchivos.mcu.es/CensoGuia/archivodetail.htm?id=1394534 There are also contact details for the individual methodist churches here:Methodist Archive site – list and contact details of Methodist churches Argentina
https://iglesiametodista.org.ar/iglesias/While there are no baptism records for William and Robert, and no 1st marriage, Walter Oldfield seems to have taken an active part in the Methodist Church at least after his wife's death. His name appears as witness to a few baptisms along with other witnesses such as the Swaneys (who were missionaries from America). Alma's parents were obviousoly protestant, while his sons were born in Rosario (according to census and other records), then it's strange they weren't baptised in the same church, which is why it would be worth checking to see if they've just been left out.
I wondered if there was any relevance to Robert having the middle name Mcdonald. This name also appears in in George William Schaub's wife's name on a baptism in 1904 Aida Macdonald Siepel de Schaub. Lots of Scots went to Argentina, so I did look at Scotlandspeople in case there was a marriage in Scotland, to no avail.
There was also an A Mcdonald who was one of the witnesses at Alma Martha's baptism and there was a Jeanie Young Mcdonald wife of J. Farquharson McDonald died in 8/10/1890 and buried in Rosario.
It would certainly be worth looking at Argentinian newspapers, as Erato says. There was a reference to a Lowe & Co. The Herald, in Buenos Aires in Mullhall's book on the argibrit site:
"Lowe & Co The Herald San Martin st Buenos Ayres page 117 in M. G. & E. T. Mulhall, Handbook of the River Plate (London: Trübner & Co., 1885) pages 669-670.
Guía Mensual de Kidd, Imprenta de J. H. Kidd, setiembre de 1884, pages 60-66, 150-151, 155."
Was there some connection with Argentina for Walter to have gone there in the first place??