PART ONE:
I have seen a few references to armoured men (brasses mostly...) with either garters and/or collars and am wondering if the collar was a sign a rank or if it primarily showed affiliation as (I assume) the garter did (<--- is this assumption correct)?

The garters seen on armoured brasses in England are very specificaly badges of the order of the garter. I would think royal perogative would prevent any other such device being worn imitatively in that kingdom.

Regarding the collars - these are livery collars, signs of personal affiliation - widespread during the Wars of the Roses. The firts such livery collar in England was the collar of SS, which pre-dates the outbreak of the war (dating to the reign of Henry IV), but marks the wearer as a committed Lancastrian - an important method of Henry to attempt to bind the peerage to his person, Given the events of Henry IV's reign (The Glendower Revolt, the atempted tripartate devision of England and deposition of Henry - all this instability brought on by Henry's violent deposition of Richard II, and the assumption - most likely false - that he had the deposed Richard murdered). At any rate, these sorts of devices are peculiar to 15th century Europe - mostly England, but the collar of the fleece (flints and fiestrykers - the fusils being intentionaly in the shape of a B for Burgundy)and the collar of St. Michael fill a similar role in French History during this time.

The other English collar is the Yorkist collar of suns and roses - for most of the period with the whie lion of march, and at the end of Yorkist kingship the white boar.

Wearing these marked one as an (allegedly) committed supporter of either the Yorkist or Lancastrian faction.

It is not a badge of rank - although they were made in gold, silver-gilt, silver, and a least a fragment has been found in pewter - they indicae a political affinity. They are intrinsicly intended for the second estate - Nobility, and the gentry, but I would be very surprised if during one of the entries into London or either King Edward or King Henry, if the burgesses of the town, and the representatives of the crafts guilds were not prominently displaying collars showing their political affinity to the king - likely the fragment I am thinking of wpuld have belonged to such a person.

Commynes mentions negotiating with the Lieutnant of Calais (Lord Wenlock - a retainer of Warwick, but who had stayed on under Edward after Warwicks exile, and so had retained his lieutenancy), just at the begining of he readaption of Henry VI, and how he was surprised to see every person in Calais was wearing a cognisance of the ragged staff - Lord Wenlock's in the form of a gold ragged staff in his hat, others wearing silver - all made up or trotted out in a very short time.

So, political rather than rank - other than the cosliness of the material of the collar - that said, John the yeoman, Tom the labourer, and Roger the cook wouldn't be wearing them - they would in the same instances be wearing quickly cut out, and pinned or sewn on badges of cloth - many cheaply made, for just such an impromtu public display of political loyalty.

These collars were principly for dress display. Although they were certainly worn in armour, and hey may have been worn in battle, they were not always worn in batle. They were indeed most often worn in ciliv dress, in daily attire.

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