The people differed on him, although the majority seem to have considered him Thiqah.
They include Yahya ibn Ma'in, an-Nasa'i, Al-'Ajli etc. Bukhari, Ibn Hibban and Ibn Khuzaimah have narrated from him in their Sahihs (where they only include authentic narrations).
His student Shu'bah has narrated from him, and it is said that he only narrated from those he ...
Your linked book cites (4) ابن هشام 2/ 655 this means page 655 of the 2nd volume of Ibn Hisham, which is likely referring to the well known work of seerah. Volumes and pages vary with the publisher, but in this case maybe the same edition is available online where you can find its chain:
قال ابن إسحاق: قال الزهري، وحدثني سعيد بن المسيب، عن أبي هريرة
— سيرة ...
They are not the same apparently.
The longer name of Maslamah ibn Abdullah Al-Juhani is mentioned in Tahzeeb Al-Tahzeeb here. Namely, his grandfather's name is Raba'ee, not Urwa.
His full name would be Maslamah ibn Abdullah ibn Raba'ee Al-Juhani.
The narrations you have quoted are not about praying to the Prophet instead of Allah, rather they are clearly praying to Allah and using the Messenger of Allah ﷺ as a wasilah.
They ask Allah to accept their supplication for the sake of the Prophet
They ask Allah to accept their supplication for the sake of their belief in and regard for the Prophet
Abu Az-Zinad أبو الزناد is 'Abdullah ibn Dhakwan عبد الله بن ذكوان a scholar of fiqh and hafidh of hadith from Medina who was born before the death of ibn' Abbas and met Anas ibn Malik and ibn 'Omar, so he is a (young) tabi' y. He died in Ramadan 130 a. H. among his prominent students you may find imams such as Malik, al-Laithi ibn Sa'as and Sufyan at-Thawry ...
Al-A'raj refers to 'Abdurrahman bin Hurmuz (عبد الرحمن بن هرمز). You can read his entry in Dhahabi's book "Siyar a`lam al-nubala", published by Muasisatur-Risalah: Volume 5, Page 69-70
Abu'z-Zinad refers to 'Abdullah ibn Dhakwan (عبد الله بن ذكوان), you can read his entry in the above reference: Volume 5, Page 445-451.
A source and a "how to"
To answer this one needs to be read, search and investigate in all available sources if Jarh and Ta'adil.
Nevertheless there's at least one book that I've learnt about that gives an attempt to answer this. It is a longer paper or letter called:
In the following and later I'll be translating from Arabic. As these ...