Nevertheless, it's firmly established in FWoaN that Galloway was back in the Congress by then, so I've got to figure out how it happened. Here's what I've come up with.
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The situation in the winter of 1777-78 was a delicate one. In spite of the defeats suffered by the rebel armies in New York and Pennsylvania, most of the people in the rebellious colonies remained beyond the reach of British authority, which was confined to the middle colonies. Any hint that the North ministry might seek widespread reprisals against the leaders of the independence movement would stiffen the resolve of those leaders to resist a return to British rule. Armed resistance could flare up at any time, leading to a resumption of the fighting and a further hardening of attitudes on both sides.
John Dickinson had always been opposed to independence, and had hoped for reconciliation with Great Britain. Now, with the fortunes of the rebels at low ebb, he saw an opportunity to rebuild the links between America and Britain. The Delaware General Assembly had offered to appoint him to its delegation to the Congress in 1777, but Dickinson had refused. After the defeats at Saratoga-Albany and Germantown, he accepted the appointment, and once in Congress, quickly took charge of the growing moderate faction. He was able to bring them around to his belief that independence had been a mistake, and by January 1778 was ready to execute his master stroke: the return of Joseph Galloway to the Congress.
Galloway had become a staunch Loyalist by the outbreak of the Rebellion in April 1775. When the Continental Congress reconvened in Philadelphia a month later, he declined to serve as a delegate. After the Congress declared American independence and the British captured New York, Galloway left Philadelphia to join General Howe, serving as an informal advisor to the general. Galloway accompanied Howe on his advance on Philadelphia, and after its capture he served Howe as head of the city’s civil government. By the time Dickinson returned to the Congress, Galloway was the most prominent Loyalist in America. 
As Dickinson well knew, Galloway’s return to the Congress would be an implicit admission by that body that the Rebellion could not be won, and that a return to British rule was inevitable. By the same token, for Galloway a return to the Congress would be an acknowledgement of the legitimacy of that body, and that its participation would be essential to the reconciliation between colonies and mother country that both men sought. Throughout the months of November and December, the two men exchanged a series of messages in secret working out the details of Dickinson’s proposed invitation, and Galloway’s acceptance of it. On January 9, 1778, the Pennsylvania General Assembly, at Dickinson’s prompting, offered Galloway an appointment as delegate; ten days later, it received his assent. Galloway arrived in York on the 29th and was seated with the rest of the Pennsylvania delegation. 
As Dickinson had expected, Galloway’s arrival was the signal for a general exodus of the most radical members of the Congress. Hancock had already resigned from the Congress shortly before the rebel defeats at Saratoga and Germantown. After Galloway’s arrival, the cousins John and Samuel Adams resigned their seats rather than remain in the Congress with him. With his influence at its height, Dickinson proceeded to carry out what has since become his most controversial act: the replacement of Washington as commander of the Continental Army.
Several of Washington’s subordinates, notably Gates, Charles Lee, and Thomas Conway, had long schemed to replace him as overall commander of the rebel army. Dr. Benjamin Rush, who was serving as surgeon-general of Washington’s army, joined in the plotting after the losses at Brandywine and Germantown. Dickinson’s motives in taking up the matter were questioned at the time, and have been ever since.  When Henry Laurens, the President of the Congress, learned of Dickinson’s plan, he attempted unsuccessfully to block it, then resigned when it became clear that he would be unable to do so. Dickinson was able to arrange for his ally Charles Carroll to replace Laurens, and his plan to restructure the leadership of the Continental Army was passed on February 13.
Under Dickinson’s plan, command of the Continental Army would be assumed by a revamped Board of War, a committee of the Congress that had been established in June 1776 to oversee military matters. A message was sent to Washington the next day summoning him to York to be notified of the reorganization. He had already been informed of the change by Laurens, whose son was a member of Washington’s staff, and when the message from the Congress arrived on the 16th, rather than travel to York in person, Washington wrote a letter accepting responsibility for the reversals suffered by the army, and resigning his commission. Washington then left the army at Valley Forge and returned to his Virginia home. 
With Washington gone, desertions from the camp at Valley Forge, already a significant problem, became endemic. Washington’s successor, General Philip Schuyler, attempted to win over the long-suffering troops, but to no avail. By the time spring came at last to Valley Forge, the Continental Army had disintegrated. 
1. Bradford Wilcox. Galloway the Loyalist, 1775-1777 (New York, 1998), pp. 288-91.
2. James Elson. Dickinson and Galloway in the Crisis Years (New York, 1901), pp. 143-48.
3. Many officers and men in the Continental Army, notably Alexander Hamilton, denounced the decision to replace Washington. Hamilton would later insist that Washington had been “the mortar that held the Army together,” and that Dickinson’s actions were meant to cause the rebel army to disintegrate. Alexander Hamilton. Farewell to Change: Thoughts on Leaving the C.N.A. (New York, 1785), pp. 51-67. For a modern version of this argument, see Christina Taylor. Sabotage: John Dickinson and the Continental Army (New York, 2011). Dickinson himself was uncharacteristically reticent on the decision to replace Washington. The Late Rebellion, Vol. III, p. 38.
4. General Sir Henry Mates. George Washington: The War Years (London, 1932), Vol. IV, pp. 556-60.
5. Ronald Coakley. A Military History of the North American Rebellion (New York, 1984), pp. 517-21.