Malcolm - I note what you say about the Gosson text - but as to early modern England not being a Calvinist country - i'm afraid that almost all historians of the early modern period would disagree with you (eg Collinson, Lake, Tyacke, Morrill, etc). The Elizabethan and Jacobean Church of England was solidly Calvinist in its doctrine and it was not until Charles I' support for William Laud that anti Calvinist elements began to dominate the Church of England. Laud paid the ultimate price for that in 1645. Further, the 'official' Cromwellian church was a blend of Presbyterian, particular baptist and congregationalist elements - all of whom Calvinists.   

Catholics were not just frowned upon - despite making up 2-3% of the population of England, they were actively oppressed, their estates seized and they were forced to worship underground. To talk openly of Catholic doctrine would inspire arrest and probably a riot and a pogrom. Most people had not ever met a Catholic but deeply feared fear them.

As to saying that references to 'our lady' 'clearly was not considered odd or inappropriate at that time' - do you have evidence?, I am a historian of the era and on my reading of this period it very much was inappropriate and odd to mention 'our Lady' - the Civil War was fought in part over the fear of popery, a fear dragged up by Charles I recruiting Irish soldiers into his Army. The Solemn League and Covenant of 1643 had declared that all who took it would 'endeavour the extirpation of Popery' from England, Scotland and Ireland - and this had been done, as all know, with much military vigour.

I have read hundreds of tracts and manuscripts from this period and references to 'Our Lady' are very rare. So therefore, the references to the 'preist was at private masse' in a public context indicates we are dealing with either pre-reformation England or Ireland, not England or Scotland from the 1560s onwards and certainly not England in the first half of the seventeenth century. In 1658 Mass had not been said in England or Scotland (except behind closed doors and with defiant trepidation) for just under 100 years.

I have had a look at the 1658 source text. The text is a book called Wit Restored (Wing Reference M1719) - the text is not an innocent broadside (if such a thing exists) but a Royalist wit book compiled by Sir John Mennes (1599-1671) a royalist vice admiral during the Civil War and and James Smith (1605-1667) a clergyman who whilst conforming to the church system of the Parliamentarian period, spent most of his time drinking in taverns, writing smutty poems and was imprisoned for debt.   

Wit Restored is a compilation of anti-Parliamentarian verse alongside smutty burlesque songs composed by Mennes and Smith, as well as numerous ballads. Little Musgrave (starts p.174) is called 'the old ballad of little Musgrave'. I would venture that it was a song Smith or Mennish had picked up from taverns on their travels and included it in their book because it was a scandalous.