I know about skelington: it’s definitely a regional dialect feature. Joseph Wright’s English Dialect Dictionary has examples of it across the country, from Yorkshire to Dorset. It’s spelled variously, such as skelenton and skillenton. Thomas Hardy uses the latter in Tess, for example, and there are two instances recorded in the OED, under atomy and know, and three examples in Wiktionary.
The Cockney use is the most famous one, and there are several stories about it, such as this one, from an online bio:
‘In a biology lesson we were shown a human skeleton and when asked by the mistress if anyone knew what it was called I shoved my hand up with some, later regretted, haste and stated quite clearly to the whole class that it was a ‘skellington’! The class erupted into paroxysms of giggling, much whispering behind hands, pitying glances and I went scarlet with embarrassment. I had no idea what I had said to get this reaction because I heard what people said, I didn’t judge them on how they said it. The mistress scathingly repeated what I had said and joined the pupils in mocking my accent.’
Or this poetic extract:
A muvver was barfin 'er biby one night,
The youngest of ten and a tiny young mite,
The muvver was poor and the biby was thin,
Only a skelington covered in skin.
Dickens has millingtary a second time - by the hairdresser in Master Humphry’s Clock (Chapter 5) – so it’s not just an idiosyncrasy of Claypole. And it turns up in several regional dialects too, on both sides of the Atlantic. Horatio Alger, for example, uses it in Randy of the River; so does R M Ballantyne in In the Track of the Troops. If you're searching, remember that there are spelling variations here too; the word often appears with a single l.
I can’t think offhand of other textual examples of an ing substitution for a short i. Has anyone come across them?