I'm glad to know you find the information useful.
"Circa 1275-1325" means an estimation for the oldest documents where "soccage" is first attested in (Middle) English. Magna Carta, as you rightly point, was first written in Medieval Latin (not English). The three times you find "sokage" in this document is in its clause #37:
Si aliquis teneat de nobis per feodifirmam, vel per sokagium, vel per burgagium, et de alio terram teneat per servicium militare, nos non habebimus custodiam heredis nec terre sue que est de feodo alterius, occasione illius feodifirme, vel sokagii, vel burgagii; nec habebimus custodiam illius feodifirme, vel sokagii, vel burgagii, nisi ipsa feodifirma debeat servicium militare.
"If any one holding of us by fee-farm or by socage or by burgage holds land of some one else by military service, on account of that fee-farm or socage or burgage we are not to have the wardship of the heir or of the land that is another's fee, unless the said [land held by] fee-farm owes military service".
Note the Germanic "k" and not the Latin "c" is used, denouncing its origin. The original word (its stem) was "soke", attested in many Germanic languages, as old as oral tradition goes (Nordic sagas) and an almost direct descent from a Proto-Indo-European root. "Soke" got into Old French presumably via Old English; the French just added the latin suffix "-age".
"Sokage" is what is "soked" (sought) by a "sokeman", just as many other gallicisms like "marriage", "leverage", "assemblage", "carnage", "sabotage", "collage", "coverage" and so on.
Gratiam habeo for such extensive explanation; I would really like to know your main sources, even if they are almost surely wrong.
Again: "Soccage" or "Sokage" is the french derivation from the Germanic (Old English) word "soke" or "sok", directly related to nowadays "seek". Nothing to do with the Romans in the dictionaries that I've checked so far.
After Hastings, England lived in an effective triglossia; Medieval Latin for the elite, a Norman dialect of the Langue d'Oil (Old French) for the nobles, Middle English for the common people.
We don't agree on this one; Feudalism (land for military service) is fundamentally a medieval (postclassic) developement.
It only got into the Eastern Roman Empire ("Byzantine" for some) as late as the XI century (Pronoia).
I would really appreciate more information on this matter and discussion too.
It would appear that land granting was part of Roman thought as they conquered lands. Did not the Romans grant land in strategic areas (usually along the rivers) to military people? A condition of the grant was to guard and defend the rivers (their highways) and when called to duty, they must return to their military group? They could settle the area but had duties to the Empire.
As for Roman agricultural legislation, I found the following book intriguing. Agriculture and agricultural practice in Roman law / Robert J. Buck. Publisher Wiesbaden : F. Steiner, 1983.
The similarities between the old Roman laws and the conditions set down by the Crown for land grants under Soccage did not go unnoticed.