I haven't heard the recording, but judging by your abc Shirley used the tune that Sarah Ann Goodyear of Axford sang for George Gardiner and Charles Gamblin (as 'Lord Ellenwater') in August 1907. Vaughan Williams re-noted it in January 1909. On an obscure website I found what appears to be a 'cover' of Shirley and Dolly's arrangement; if that is what it is, then Shirley didn't use Mrs Goodyear's text (which was incomplete; she had forgotten some lines), but a slightly modified form of a rather longer one from Sussex that, as it happens, I posted in another discussion here some years ago: Help: hopeful for Lord Ellenwater info.

I can't say I like your proposed ending; it quite spoils the dropped close, which as Bronson pointed out gives a mixolydian feel to what is otherwise a major tune. Here is what Mrs Goodyear sang, as noted by RVW:

X:1
T:Lord Ellenwater
S:Mrs Sarah Ann Goodyear, Axford, Hampshire. August 1907 & January 1909.
N:Tune noted by Charles Gamblin and Ralph Vaughan Williams.
N:Text noted by Dr George Gardiner.
N:Variations in bars 3 and 6 omitted.
B:Journal of the Folk-Song Society, III (13) 1909, 270-1.
L:1/8
Q:1/4=100
M:4/4
K:G
D | G3 F G2 B2 | AG E2 D2 G2 | AB cA d2 cB | A4 z2 G2|
w:The king he wrote a long_ let-ter And sealed_ it_ up with_ gold And
A B (cA) d2 e2 |d B (AF) G2 F G | A3 B (AG) (E^C) |D4 z2 z |]
w:sent it un-*to Lord El-len-wa-*ter For to read it if_ he_ could.

It may be that all you need to do in order to provide a suitable variation for the last line is to restore Mrs Goodyear's sharpened C in the penultimate bar.