Hi Kylie
I do think you are on the right track, however you may never have proof as some of the baptisms and marriages you mnetion are simply not in the OPR records. You could make up a tree to use as a basis for further research, as follows:
You have Thomas Leiper who married Marrion Biggar 4.4.1776 in Avondale parish, Lanarkshire. This marriage is in the records.
Thomas had a son Thomas Leiper b. 2.4.1778 Bapt. 8.4.1778 Avondale parish Lanarkshire. This baptism is in the records, altho' mother's name is not shown.
I think it is possible that Thomas was b. Avondale (town of Strathaven) as per this baptism record, and not in New Cumnock. Because Thomas married in New Cumnock, his son Robert Leiper may simply have assumed that his father was born there. Especially as Robert's older sister Jennet was born in New Cumnock. Here I am assuming that the only reason you say Thomas was b. New Cumnock is that this was stated on Robert's immigration records. Is that right?
Thomas married Agnes Herbert 1797 in New Cumnock, but this marriage is not in the OPR records. Thomas and Agnes had 8 children as per my list in the previous 2 posts. The first child was b. in New Cumnock, the second, Robert, in 1799 Paisley, Renfrewshire.
I am inclined to think this was basically an Avondale (Strathaven) family. Like so many Scottish families they had to move away to find work (Thomas was described as a 'day labourer'). Thomas b. 1778 went to New Cumnock where he met and married Agnes Herbert and they had their first child. They must have immediately moved to Paisley, Renfrewshire as their 2nd child Robert was born there in 1799. Soon after, like hundreds of thousand others, they went to Glasgow where all the work was. Robert returned to Avondale to marry Isabella Miller in 1828. (altho' this marriage is not in the records.)
Have a look at a map and you will see that the distances we are talking about are not great.
I will get back to you if I find anything else, but I do recommend you get onto Scotlandspeople yourself.
Diana