Where dj'a get this spelling from, dubdub? Looks interesting.

fwiw, I would go with the Welsh way of pronouncing the 'wes' part - treat the W as we name it, which is to say a long 'U' sound, the e functioning more as a silent case marker with the s marking it plural. This makes the overall sound “marsh~me~looze”, or very much similar to our current 'marshmallows'.

b. A confection made from the root of this plant. Also fig., esp. something or someone that is soft at the centre, ‘gooey’, sentimental.

1884 Chamb. Jrnl. 4 Oct. 640/2 Gum-arabic+makes about thirty per cent. of the best quality of gum-drops, marsh~mallow, and jujube paste. 1902 Westm. Gaz. 11 Aug. 12/1 Chocolates, ‘marsh-mallows’, and other toothsome delicacies. OED2