Are manual tyre changers any good ?

[Andrew Shadow]

It turned out to be corrosion around the inside of the rim - due, apparently, to using tyre sealant gloopy compound.

More to the point the corrosion was probably caused by not using tyre sealant gloopy compound. A lot of places just use soapy water to lubricate the tire bead for installation which promotes corrosion of the bead seat. Proper tire mounting lubricant does not promote corrosion.