Well without the letters, would the Americans rebel but in even less numbers or would they be the same?
I'm not sure what you're referring to here. But I think it would mean that Boston and Massachussets are slightly less radical, or at least their radicalization is slowed.
Also, would there be potential of Galloway's plan of Union passing with a less inflamatory buildup.
With many of the events, would it be possible for the vote to be reversed, and his plan being narrowly accepted rather than narrowly defeated (I think it was just one vote that sent it down the drain).
Could this plan of Union, a still present Hutchinson, and maybe even Ben Franklin come together to create a new plan?
It will take a bit more. [And neither Hutchinson nor Franklin were delegates to the First Contitental Congress].
The immediate effect, I think, is to alter the train of events which led to the Boston Tea party. Here's a possible chain of events:
With the demonstrations surround the controversy of the letters, then the Sons of Liberty have gone a bit longer with no obvious evidence of tyranny lurking in the halls of government and Thomas Hutchinson has had no reason to crack down.
However, the Tea Act is probably still passed. Hence, HEIC ships arrive in Boston Harbor (as they did in Philadelphia and New York). In the latter two cities, citizens refuse to purchase any tea and the governors allow the ships to leave without landing any cargo to avoid incidents (thus allow no import tax to be paid).
OTL Thomas Hutchinson wanted a showdown with Samuel Adams and ordered the HEIC ships to disembark their cargo (and thus that the tax be paid). Hence, Adams put together a group and did jus that...by chucking them into the harbor. Without the events of OTL 1772, I'd imagine that Hutchinson is less on edge.
Samuel Adams and John Hancock still don't want anything to do with the tea (or its tax) so they stage a protest: they gather a large group of followers along the docks of Boston Harbor (or maybe on the Common) and have a big party. They are having what appears to be a Tea Party, but they are drinking only Blackberry Ruminant (the colonial subsitute). They do this in mid to late November, before Hutchinson has had time to insist they pay the tax. Many during the demonstration remark how horribe the substitute tastes and Boston Commons 'runs brown' with drained ruminant. This ruins any enthusiams for a latter public meeting Adams has been planning to officially refuse to pay the tax.
Upon receiving a letter from Benjamin Franklin urging that no property damage be permitted against HEIC wares, Hutchinson hatches a scheme: he will take out a loan on his Milton estate and pay the import tax on the cargo from one British ship. He will then call a Royal Party for Christmas Day 1774 and he will use real tea. Mild scuffles continue throughout December, but nothing on the scale of OTL. Hutchinson's party occurs. Samuel Adams' Sons of Liberties disrupt the proceedings dressed as Indians. In the tumult, Hutchinson is knocked from a platform, breaking his arm; his wife is also hurt (her ankle twisted) though many suspect that this was due to tripping over Hutchinson. The Loyalists now have a perfect standard bearer for Law and Order in the colonies, though.
The same protests and declarations in the other colonies occurs on schedule.
Parliament still passes some Acts which gives the colonists room for pause. They pass the Government of Massachusetts Act, which increases the power of the Governor to call for order, but does not enact the sweeping changes of OTL. The Administration of Justice Act, Quattering Act, and Quebec Act are also passed, but the Boston Port Act is not passed (nor will the New England Restraining Act).
The First Continental Congress convenes in 1774 to discuss the "Questionable Acts." They discuss a plan of Union. They colonies are deadlocked, but they decide to send it to Benjamin Franklin in London. [The Suffolk Resovles do not disrupt debate on the Plan, but it is not unanimously adopted because of concerns about its form]. A Pettion accompanies it, endorsed by the several colonial assemblies (i.e. by the representatives of those assemblies in congress assembled, rather than by the Congress itself).
In London, William Pitt and Benjamin Franklin attempt some sort of compromise on the basis of the Galloway plan, but all Pitt's attempts are defeated (because of reform issues they engender). They don't fail quite so badly as OTL, however.
The Continental Association goes into affect to further the boycott of British trade. Many grumble.
Franklin presents the Petition of the Colonies to North. He attempts to dissmiss it as improperly formed, but can't. It is in turn present to George III. All parties agree something must be done, but no one agrees what.
By late 1774, things are growing tense. Hutchinson writes to Franklin stressing the urgency of events. The Sons of Liberty are continuing to act freely and he fears for the order of New England. The antics of the Sons of Liberty causes some dissension between John and Samuel Adams. John writes another treatise on the rights of Englishman while Sam's Sons of Liberty have discovered they quite like gallavanting across the Common dressed as Indians.
By February 1775, North relents and urges the passing of the Conciliatory Resolution which abolishes taxes for any colony that promises to pay for the "common defense, support for the civil government, and the administration of justice." North prefers this to Pitt's system of mutual concession: Parliament must not shirk its rights, he believes. He does not however move to declare Massachusetts in Rebellion.
South Carolina jumps at the Act, as does New York. By years' end Pennsylvania has followed as does New Jersey.
Massachusetts and Virginia are still prominent hold-outs. Samuel Adams and Patrick Henry continue to call for clarification of the rights of the colonists as Englishman.
I'd go one, but I think from here there's still a chance fighting might break out, but more likely the newly invigorated Loyalists have a chance to find reform. It will probably take until 1780 or so, but by that time the cause of Reform in gneral is much enhanced. Edmund Burke's speech on Conciliation provides a template which is a middle way between outright Parliamentary representaion for the colonies, colonial union, and the status quo. The colonies are still rambunctious places: Indian Affairs and colonial settlement will be huge issues. And there's a reckoning between Britain and the Continent brewing that has not yet broken out.