Those aren’t “incorrect”. They’re now regional rather than standard.
Outside of Scotland, they mostly haven’t been seen nor heard since the American Revolution. There are some 19th century examples from Scots writers, however.
Regarding shew (past of show), the OED in particular states that:
The spelling shew, prevalent in the 18th c. and not uncommon in the first half of the 19th c., is now obs. exc. in legal documents. It represents the obsolete pronunciation (indicated by rhymes like view, true down to c 1700) normally descending from the OE. scéaw- with falling diphthong. The present pronunciation, to which the present spelling corresponds, represents an OE. (? dialectal) sceāw- with a rising diphthong.
The latest citation I could find for that sort of spelling antedates the American Revolution, albeit by a scant two years from when it was written (although not when it was published):
- 1774 Goldsm. Nat. Hist. (1776) V. 210 — A partridge is shewn him, and he is then ordered to lie down.
From Guy Mannering or The Astrologer, Scotsman Sir Walter Scott writes in 1815:
- 1815 Scott Guy M. x, — The chase then shewed Hamburgh colours, and returned the fire.
Scott also used that spelling for the noun:
- 1789 Scott in J. Haggard Rep. Consist. Crt. (1822) I. 13 — It often happens that on a shew of hands, the person has the majority, who on a poll is lost in a minority.
Shakespeare normally used show, but in one noun instance he used shew:
- 1611 Shaks. Cymb. v. v. 428 - As I slept, me thought Great Iupiter vpon his Eagle back’d Appear’d to me, with other sprightly shewes Of mine owne Kindred.
And no one less that Charles Dickens wrote in 1840:
1840 Dickens Old C. Shop xvi, — ‘Good!’ said the old man, venturing to touch one of the puppets,.. ‘Are you going to shew ’em to-night?’
It has an old-timey feel to it.
Saying glew is something else altogether. It is either much older and in Middle not Modern English, where it showed clear derivation from its Old English ancestor:
C. 1000 Ælfric Saints’ Lives vii. 240 — Þæt fyr wearð þa acwenced þæt þær an col ne gleow.
A. 1400 Isumbras 394 — Smethymene thore herde he blawe, And fyres thore bryne and glewe [rime ploghe].
Notice the mention of the rhyme with plow/plough in 1400.
So your glew is either pre-Modern English, or else it is a different verb altogether, and in the present tense. There’s one from glee:
- 1. intr. To make merry; to jest; to play on musical instruments.
- 2. To call loudly on.
- 3. trans. To afford entertainment or pleasure to; to make happy.
And another that appears pseudo-archaic from glow with the sense:
Both those glew verbs are now considered obsolete in Standard English.
Your what-if is immaterial and unanswerable. These things happen all the time. Just because two different regions have two different ways of inflecting a verb doesn’t make one of those regions “wrong”. One or both — or even neither — may simply not be Standard English.