Okay here's a summary of 'Cassillis' appearances in 18thc versions.

TTM has no mention but as you say that could easily have been for politic reasons. V1 has 'good lord's gate' and 'fair lady' and last line is 'fair young wanton lady'.

SMM has v1 'Our lord's yett' and 'the fair lady' but last line 'The Earl of Cassilis' lady'

Mansfield has v1 'Lord Cassillees gate' and 'fair lady' with last line 'The Earl of Cassillees lady'.

The Roxburghe slip has v1. 'Earl of Castle's house' and 'Earl of Castle's Lady' and last line 'Earl of Castle's lady'. As Ebsworth rightly states this printing is full of Englishisms like 'house' for 'gate'.

All versions are very close in text but Mansfield has the extra verse found in a few later versions.

v4
Aft have I ridden this wan water
And a' my maids beside me
But now I maun wade and weet my feet
Whatever may betide me.