I wouldn't say he was "big on the attainment Buddha Nature" as he only uses it in one paragraph when not found in various quotes.
Sure he was big on Buddha Nature,as your notes show almost the entire 5th chapter is based on Buddha Nature and how one will attain Buddha Nature/Enlightenment in the Pute Land,this is to show that the GOAL for going to the Pure Land is to acheive Buddha Nature/Enlightenment.
Also 99% of the Kyogyoshinsho is QUOTES and only 1% is Shinrans commentary on the subject,this is because Shinran let the sutra passages and quotes from other masters speak for themselves,then he only added alittle commentary to clue you in on what the TOPIC was.
The "various" quotes as you say is the actual teaching and the purpose of the Kyogyoshinsho,Shinrans actual commentary was to set up the TOPIC then elaberate on the quotes provided for the topic.
You say that he only used Buddha Nature in one paragraph when not found in actual quotes.......well if you look at chapter 5 Shinran only speaks 3 paragraphs (1) is his presentation and has nothing to do with elaboration on the teachings.
Verse(37) is his teachings on Buddha Nature(confirmation of his numerous quotes)
Verse(39)is on true/provisional buddha/buddha land
So as we can see in the context of his writings on the 5th chapter he only has TWO paragraphs of commentary, and as you said one paragraph concerns Buddha Nature, this means HALF his commentorial writing in chapter 5 concerns Buddha Nature.
And since Shinran taught not through personal commentary but through direct quotes,this means 90% of chapter 5 teachings concerns the establishment of the Buddha Nature teaching and its attainment in the Pure Land(ultimate goal of rebirth in Pure Land) very important things.
The quotes are used in various contexts a are not related to one's practice or anything that one should understand now. Buddha-used as a synonym for various things (buddha qualities, compassion, liberation, potenti enlightenment, buddhahood) and not defined as anything central to the doctrine of Jod Shinshu.
Sure those quotes and teachings from Shinran are central to the doctrine of Jodo Shinshu....if they wasnt central doctrine then Shinran would of never wrote an entire chapter on the subject in his Kyogyoshinsho to begin with.