The entry from the catalogue of the NRS which you posted gives the fiar of Ambrismore as William. The testament and inventory abbreviates his name to wa, hence my tentative Walter?, since William is usually abbreviated to Wm. I have changed my transcription of the last page, but it still says wa[lter?] in the notes/corrections to p 5 and I can't change them now.
There is nothing really odd about High Boyd being referred to as son in law of the defunct. Three different people have been involved in the writing of the document and the format you have here is not the original papers, but the entry of them in the court books. The fact that it is written in a less than orderly way is probably due the number of scribes.
On closer inspection, the name at the end of the 14th line of page 5 is "S(i)r James stewart".
In Scotland the word used is confirmation, rather than probate, which tends to be an English term.
The Ninian Stewart elder of Kilchattan mentioned at the end is there because he he has become cautioner for the executor. He cannot, therefore, be the same person as John Strewart of Ascog's eldest son Ninian, as he is the executor.
The reference to cautioners in the first of the two NRS records is not a reference to cautioners for the executor of the testament, but as cautioners for the bond.
The place-name is Denune = Dunoon. The crescent mark above the word was often used over the letter u in the same way that a dot is used over an i or j.
I looked through the debts owed to John Stewart (starting near the end of page 2) and these are Stewart references extracted from it:
p 2
Item y[ai]r (= there) was awin to the s[ai]d vmq[uhi]le Johne stewart Be niniane stewart of [a filler, not a place] [con]forme to his [blank, but would refer to a bond] Lxxx lib.
Item be (= by) w[illi]a[m] stewart fear (= fiar) of ambrismoir j c xxxiij lib vj s viij d
Item be (= by) [no first name given] stewart of kilhattin [con]forme to ane compt (= account) j m iij c xxxiij lib. vj s viij d
p 3
Item be (= by) Jon stewart oy (= grandson) to the defunct vj c xxxj lib. xiiij s viij d
The reference to the defunct is the same as for Hugh Boyd.