Joseph SCOTT of Kirkland Bachelor married Sarah MARR of Kendal spinster in Holy Trinity, Kendal on the 01 May 1786.
Given that John and Agnes MARR were living with Joseph & Sarah SCOTT in Stricklandgate at the time of the 1787 census, I would assume that this is the correct marriage, especially as they appear to be Sarah's siblings.
A Sarah MARR was chr. 12 May 1765 in Holy Trinity, Kendal, the d/o John & Mary MARR of Stricklandgate. I haven't checked the registers for her siblings, yet the IGI lists William (chr.30 Jan 1763), John (chr. 30 Nov 1766), Agnes (chr. 25 Dec 1768) and Thomas (chr. 14 Jul 1771).
John MARR maried Mary SUMERS, both of Kendal, in Holy Trinity, Kendal on the 17 Feb 1761.
The two Joseph SCOTTs baptised in Kendal that you mention were
Joseph s/o William & Elizabeth SCOTT of Kirkland, chr. 13 Jan 1765 Holy Trinity, Kendal
Joseph s/o John & Jane SCOTT of Stricklandgate, chr. 31 Aug 1766 Holy Trinity, Kendal
The family of John & Jane SCOTT appear to have lived on Stricklandgate from at least 1766 (Joseph's baptism) until the time of the 1787 census. Therefore, given that the Joseph who married Sarah was of Kirkland at the time of his marriage (1786), I suspect that he is more likely the son of William & Elizabeth. If this is the case, then the naming of Joseph & Sarah's children (Mary, Wm, John,..,Betty) after their parents seems quite plausible.
If the above is correct, then Joseph's parents appear to be
W[illia]m SCOTT of Kendal & Eliz[abe]th BUSHER of Kirkland who married in Holy Trinity, Kendal on the 30 Sep 1759.