You have to remember, that to many British subjects in Great Britain, this was not a complicated question- The American were rebelling against their sovereign and rightful government. British soldiers being fired and killed at Lexington and Concord was an act of treason, as far as they (and many other people) were concerned.

So, to your second point- there are lots of sources that document how rank-and-file British soldiers felt about the American Revolutionary War. From pamphlets, private journals, news reports, and cartoons and caricatures, we can get a picture of British soldiers who disdain the rebellious colonials and their cause, proud of their status as Britons, and held together by a strong Esprit de Corps.

Firstly, many British soldiers saw the colonial cause as hypocritical and overblown. A good example of this opinion is the pamphlet Taxation No Tyranny by the writer Samuel Johnson, published in 1774:

"In favour of this exemption of the Americans from the authority of their lawful sovereign, and the dominion of their mother-country, very loud clamours have been raised, and many wild assertions advanced, which, by such as borrow their opinions from the reigning fashion, have been admitted as arguments; and, what is strange, though their tendency is to lessen English honour and English power, have been heard by Englishmen, with a wish to find them true..."

Or,

"When subordinate communities oppose the decrees of the general legislature with defiance thus audacious, and malignity thus acrimonious, nothing remains but to conquer or to yield; to allow their claim of independence, or to reduce them, by force, to submission and allegiance."

And the most commonly quoted passage of it:

"We are told, that the subjection of Americans may tend to the diminution of our own liberties; an event, which none but very perspicacious politicians are able to foresee. If slavery be thus fatally contagious, how is it that we hear the loudest yelps for liberty among the drivers of negroes?"

Caricatures and cartoons were also cheap and commonly distributed- This was the British golden age of satirical prints. As you can see, in examples here a cartoon mocking the Continental soldiers after their defeats around Philadelphia in 1777 (In this case, done by a British officer, Richard St. George Mansreagh). Other cartoons celebrated British successes, depicted the Americans as savages, and mocked their alliance with France and other European nations. They also heavily lamented the eventual peace treaty. These would have circulated in Britain and been accessible to a wide audience. These cartoons often play upon British nationalism and portray the war was a fight not just against the rebellious colonies, but Britain's recent enemies- the French and Spanish.

There's also more than a few oblique references by British officers and soldiers in their private journals and public accounts of their attitudes towards their enemies. One good was made by an officer recounting the battle of Harlem Heights: "...[The battle] presented an opportunity of showing the difference of British and American spirit. Everyone of the enemy's killed and wounded stunk infamously of rum. Their canteens still contained the remains of sheer spirits, even their officers were in this manner urged on...". This charge would be leveled again at the Battle of Bennington the next year.

Lastly, many British soldiers were motivated by the very human urge to protect and gain the esteem of their immediate comrades: Namely their fellow soldiers. Matthew H. Spring's With Zeal and Bayonets Only: The British Army on Campaign in North America 1775-1783 has a good analysis of this. He points out that many British soldiers that embarked for Boston in 1774 and 1775 had a majority of soldiers who had served for at least 5 years, and some many more. These regiments weren't entirely static, but many of the enlisted men would have felt strong attachment to their fellow soldiers and immediate officers.

Works cited:

Spring, Matthew H. With Zeal and Bayonets Only: The British Army on Campaign in North America 1775-1783 (University of Oklahoma Press, Norman, OK, 2008.

Johnson, Samuel. Taxation no Tyranny. [https://www.samueljohnson.com/tnt.html]

Another slightly dated, but still good book, on this subject would be this one:

Berger, Carl. Broadsides & Bayonets: The Propaganda War of the American Revolution Revised E. (Presidio Press: San Francisco, 1976).