To think of Rasulullah ﷺ in Salah is worse than thinking of cows and donkeys.
The above mentioned statement is not found anywhere in 'Siraate Mustaqeem' that, 'To think of Rasulullah ﷺ in Salah is worse than thinking of cows and donkeys.'
The actual word which was written in the ibarat was 'Sarf-e-Himmat'. This is terminology used by the Sufis in Tasawwuf.
The book, 'Siraate Mustaqeem' is based on 'Tasawwuf'.The objector is not versed in Tasawwuf'. Therefore, he translated 'Sarf-e-Himmat' to mean a mere thought.
'Sarf-e-Himmat' in 'Tasawwuf' means that a person's meditation over a thing becomes so overpowering and predominant that no other thoughts penetrate into the mind and soul.
To contemplate over a figure so that no other thing is contemplated is called 'Sarfe Himmat'.
This has been forbidden in Salah, that besides Allah, 'Sarf-e-Himmat' should not be done towards anyone. Salaat should purely and solely be for Allah alone. If 'Sarf-e-Himmat' is done towards Rasulullah ﷺ, then the entire Salah and Ibaadah (worship) will be for him.
On the other hand, if any thoughts of cows, donkeys, business, etc. come to mind, or a person gets drowned in these thoughts whilst in Salaat, it is regrettable. There is no fear of it being worshipped.
In fact the person regrets that in the course of an esteem Ibaadat like Salaat, he should have such thoughts, Astaghfirullah.
What comes to mind is this; that the Objector presents a picture of a Grade One child, learning to read and write ABC and wishing to interpret the writings of Shakespeare.
When the name of Rasulullah ﷺ is recited in 'Tashahhud' the thought of the Rasul ﷺ will come and should come. The Salaat will not be rendered incorrect and this is not unlawful at all. The respected Moulana did not stop anyone from this.