I looked around my ethnographical journals. From a couple of articles as well as Google searching I found only a patchwork of information, so I'll only tell you what I was able to synthesise from it.
The custom is indeed ancient. Modern Western customs seem to have ancient Roman tradition as their common ancestor, but it was already widespread at least among cultures which left written evidence, which dates to late Roman Republic at the earliest as well as Ancient Greece during its classical age, 5th and 4th century BC.
Roman marriage customs seem to have endured and spread along with Christianity, which is no surprise as marriage is one of the sacraments. Romans sealed the marriage with (iron) wedding rings, for example.
The best man's duties were concerning various legal matters over transfer of
potestas from bride's father to the groom (in marriage
cum manu), dowry, escorting the bride to groom's house, act as a witness etc.
As for Greeks, I found a curious word for it,
paranymph (παράνυμϕος). It has its own
Wikipedia article.
Thankfully, the New Testament was written in Koine Greek, so I found that John the Baptist in John 3:29 refers to himself as "the friend" of the bridegroom (Jesus, his bride being the Church)
"The bride belongs to the bridegroom. The friend who attends the bridegroom waits and listens for him, and is full of joy when he hears the bridegroom’s voice. That joy is mine, and it is now complete."
Various commentaries, as seems easy to discern, explain his role as groomsman (Meyer's NT commentary) :
The friend of the bridegroom (κατʼ ἐξοχήν: the appointed friend, who serves at the wedding) is the παρανύμφιος, who is also, Sanhedr. f. 27, 2, called אוהב, but usually שושבן. Lightfoot, p. 980; Buxtorf, Lex. Talm. s.v.; Schoettgen, p. 335 ff.; and see on 2 Corinthians 11:2
From there I took a look into Hebrew customs, which is difficult due to mostly antisemitic sources. I'll give an example from the 19th century Encyclopaedia Metropolitana:
The duties of the Hebrew Paranymphs, שושבן, were so revolting and unmanly, that nothing but that inveterate prescription, which so often divests national customs of unseemliness in the eyes of those to whom they have become familiar, could have reconciled a comparatively civilized People to their toleration and practice. We dare not quote, even in a learned language, the very curious, but somewhat too broad particulars, upon which Selden has ventured in the XVIth Chapter of the IId Book of his Uror Ebraica. (Opera, ii. 636.)
However, I've found that the word mentioned,
שושבן, denotes best man in an article "Susapinnu: The Mesopotamian Paranymph and His Role". This concerns Akkadian (also Semitic) culture. I no longer have Jstor access, but if you know someone who does, on these forums perhaps, you can check it
here.
In other scattered articles, essays and sources, I've only found various local and regional flavours of the vastly similar custom of person outside the blood relative group entering a formal relationship between two families in a sort of spiritual kinship. Reading about customs in Croatia, I've found numerous sources claim that some families have for over hundred years or more offering and expecting best men from each other, in a form of interfamilial alliance.
All this leads me to believe that some form of custom, or its early form, is as old as recognition of kinship, as old as families, although of course, more easily found where a family is able to establish roots, that is, with the advent of agriculture after the Neolithic Revolution around 12000 years ago. In any case, if the earliest possible written sources (Mesopotamia) contain mentions of the practice, it's safe to assume that it existed way before.
As for your initial question on groomsman taking care of the widow, it may very well be the case, and expected, due to strong familial bonds. However, here we enter the territory of local custom which may wildly differ as close as the next village. Waylander mentioned a story about bride taking the best man as husband after the bridesgroom absconds. If there is a strong bond between two families or clans etc. who are expected to marry between each other and obliged to be each others' best men, as used to be the case in scattered locales in Europe, it would be no surprise at all.
About Goths and Romans raiding villages for women, it's as close to truth as ius primae noctis, Columbus sailing west in order to prove that the Earth is not flat, notion that Stone Age people grunted and seduced women with stone clubs. At best, it's fanciful bullshit, most of which we were infected with by 19th century. It holds that all history before us consists of primitive people being stupid for our amusement or to make us feel superior. I'm sure it will spread inside the blogosphere and eventually spill outside, but I do recommend that you read such sources with assumption that they are wrong. Then go and verify.
The source of this factoid is a book titled
Save the Date: The Occasional Mortifications of a Serial Wedding Guest by Jen Doll, a humorous "chick lit" memoir described by a reader as "bitchy". Not the best source, really.