One use of the comma in English is to indicate a slight pause; so there is a difference between "Roses are red and violets are blue" and "Roses are red, and violets are blue" - the latter has a slight pause before "and".
In the same way I would suggest the comma before "respectively" marks a slight pause.
One can in fact say it either way.
The only corollary to this is that in recent years commas have been dying out - I use them only where necessary these days to make the meaning clear or to clearly indicate a slight pause.

I stumbled on the following at

http://www.writing.utoronto.ca/advice/english-as-a-second-language/expressions-of-quantity

"Expressions of time, money and distance usually take a singular verb:
Ten dollars is a great deal of money to a child.
Ten kilometres is too far to walk.
Six weeks is not long enough."

@AnWulf Ah "bod" - a word from my childhood: not really a word I would use in print unless in the wordstring "an odd bod", but wellworth mulling over.
Re your remarks about the OED, I must say that English wordroots are quite muddled and muddling - more Latin in OE than at first sight, more Frankish in French-rooted words, more early Latin borrowings into German. I really think the bod-in-the-street will never know the sundriness of it all, nor know which to choose.
Again, if there were Latinate borrowings into OE before 1066, then one would have to allow some borrowings in the following yearhundreds as an everyday happening where tongues are brushing shoulders so to speak.
Thirdly, our gripe is more about the unneeded academic doublers like mortal,lethal,fatal instead of deadly; so there is no need to uproot short French borrowings like "joy" and try to bring back "frothe", which is no shorter nor in today's world less English.
Out of all this, I would put forward a deem-standard grounded much more on what sounds snobby or academic in today's English, rather than grounded on word-roots.
For instance, the French/Latin birthing of "agree" is scarcely noticeable to a French bod today, so why not just take it in as English, be it a natural or forced borrowing or not. Whilst wordroots may hold you and I in thrall, they and their ilk are of little use to everyday folk.
And lastly there is the slow death of English dialect words afoot today, and I wonder what can or should be done about it, if anything. I do fear that in the end we shall all have the vocabulary of Rambo. The other noteworthy thing is that in England at the last tally there were over three hundred primary schools where not one child had English as their mother tongue. What this bodes for English to-come I dread to think.
:)

On a slightly different topic, is Mexico in North America or Central America?

@Rosewood11 You might wish to take a look at the "anglish" blog on this selfsame websheet, wherein, amidst much wailing and gnashing of teeth, your KJV-standpoint would be most welcomely brooked.

Yes, very much an EFL issue, where one may need to explain why "on an assisted passage" is okay, but "on a helped passage" is not; or why one cannot say " a disappeared species" (but perhaps "a vanished species"), "resulted in decreased production" (but not "decined production"). Sometimes it seems to hinge upon whether there is an underlying passive idea, whether one understands "a withered plant" as "a plant that has withered" or "a plant that has been withered". Othertimes it seems just idiomatic such as "the data provided" (but not "the provided data").

@WW Thanks. The tricky thing in English is the way so many intransitive verbs have a slight change in meaning to causative when used the passive - thus "withered" really means "made to wither" in the passive; but in 'a withered arm' seems more of a middle voice to me.
Actually the reason I brought this up was in Korea most students were/are taught that the "passive is more formal" and then use it regardless of meaning eg "What was happened?". In Korean passive and causative are often morphologically the same so confusion is not surprising. Hungarian is similar with an English passive often equating to a causative or reflexive plus middle voice. The next hurdle is in which context one can use a past participle as an adjectve.
In the end grammar can only take us so far and the rest is learning the English idiom.

"should" here is somewhat academic. There are millions of English speakers north of St Albans who rightly use "he were" and "we was" as a relic of Viking grammar - take a listen to "Chlorination Street". One could blather on about subjunctives and the kinship to "waere" in German; but it's all pretty much undermined by those Viking offspring. So don't get ye knickers in a twist about it.

@Ængelfolc: Thanks. The words you list are fine for true-to-life meanings; I was thinking more of "flexible working hours" or "The boss is very inflexible when it comes to pay rises", where the meaning is carried over to another realm, (like 'flexibel').
On another point, there are two types of yoga teachers: those who say "inhable/exhale" and those who "breathe in/out". The ask is why do they choose one over the other? Sometimes I think is is easier to stress that first wordlimb 'in' or 'ex' so the instruction is clear. Sometimes I think it is just unawaremess and booklearning.
Nobody uses 'hale' instead of 'breath'.

@WW What bugs me sometimes is the "open-door" policy of English - yes I know this gives us lots and lots of words to choose from, but there is a flipside when it comes to non-natives learning English. For example: in Hungarian, town/ciy is "varos", "fo" is head; so the "capital" is "fovaros". It's pretty straightforward. (There many borrowings in Hungarian but quite where they came from is often a mystery, but seldom latin). Meanwhile in English we have the word "capital" (which of course comes from caput capitis a "head" in latin, although one wouldn't know that unless one had learnt latin at school). So whilst the Hungarian word is guessable from its roots the English one is not, (unless one's L1 is romance). So in the end the "open-door" policy leads to a mass of words whose root meanings are obscure. I think we as native speakers just acquire these words their usage as "blanks"; I certainly don't think of "in-fer", "de-fer", "re-fer", "of-fer", "suf-fer" and "pre-fer" as prefix+"fer" meaning to carry, (but nothing to do with "fer-al", "fer-ocious"); whereas in Hungarian and some other languages the roots would be clearer to native speaker and outside learner alike.
It is of course a bit late to change all that or to speculate what English might have been like if Harold Godwinson had fought a bit harder. What is done is done. And in some ways what a fine mongrel tongue we have. And in other ways we still have this lingering part of our culture that often tends to put latinate words in the "high" register, French in the "neutral" register, and the rest is just wot everywun says, like a reflection of the class system that operated in England when I was young.