Don’t forget verse 33/33 is for wives FIRST whom Allah swt promises purity!!!
Surah Ahzab switches addresses all over. I can go into details of this through out Surah or you can observe it yourself.
But the way wives are addressed is similar:
وَاتَّقِينَ اللَّهَ ۚ إِنَّ اللَّهَ كَانَ عَلَىٰ كُلِّ شَيْءٍ شَهِيدًا
إِنَّ اللَّهَ وَمَلَائِكَتَهُ يُصَلُّونَ عَلَى النَّبِيِّ ۚ يَا أَيُّهَا الَّذِينَ آمَنُوا صَلُّوا عَلَيْهِ وَسَلِّمُوا تَسْلِيمًا
The line before God telling us to bless the Prophet is addressed to the wives to fear God. What is the relationship and flow?
It's that they should fear God especially that they are married to a person who God his Angels and believers all bless.
Do you understand and agree?
Well, 33:33, is telling them stay in their houses as in don't leave the Rasool (s) for the Dunya as that was a choice given to them, but fear God instead and to obey God and his Messenger.
So to encourage them to choose Rasool, it emphasizes he is purified by God thoroughly and is intensely pure. Having mentioned the Ulul-Arham in 33:6 to be our Awliya in same way Nabi is awla to believers, as well having mention as wives being our mothers, it wants to clarify all this as well. While his Ahlulbayt (a) share him in the status of being attributed purity by God and chosen by that to represent his religion, and God attributes purity to who he pleases and is not for humans to do so, it's obvious the wives have to choose to be with Rasool (s) over Dunya and their choice is not a guarantee. Thus while they are our mothers, they are being emphasized to take that role seriously and they will be punished more then others if they fail and rewarded more then others if they pass, mainly, by the fact they are married to Rasool.
Before that it was also emphasized that Rasool and wanting him is linked to wanting God and next world.
So same way, mentioning position of Salawat with respect to them fearing God, mentioning the total purity of the Nabi, is to encourage them to choose him over dunya and the degree they should fear God with respect to him.