A professional translator may well be able to translate the average light novel in 1-1.5 months but that assumes that they are not working on other things as well, Yen will not have an infinite amount of resources and this is not the only title they will be translating. Also the translation portion of the production is only a part and the other parts of the production will also take resources which need to be shared with other releases as well.
A fan translation may also be quicker than a professional translation due to the lower standards of quality required in the output. Yes, I have read some fan translated novels and some of them were well done but others were quite poorly translated and frequently difficult to read, (almost as bad as my spelling and grammer

). The well done ones tended to be released at a chapter, not a novel, a month at the fastest rate.
I can't agree with the comparison with western novels though. I tend to read pretty quickly, as such I read the Spice and Wolf book in a day. According to the schedule I should have the next volume in six months (and volume one of the manga in April) and so on for ~7 years for the current 14 books. Western books may be longer, but I have got authors I collect who have released a volume a year for significant numbers of years (I'm thinking 10 volume series in some cases). It, depending on free time, may only take me 3 days to read a volume of their work, then I have to wait a year for the next volume. With this release it may only take me a day to read it but in six months I have a new section of story.
To many releases per month can mean that the market becomes saturated thus affecting sales, something which appeared to happen to R1 anime releases a while ago, hence increasing the release rate may impact Yen's many other releases hence will be a factor in the release pattern they have decided on. Yen may be trying to increase their sales on this title into the generic Fantasy genre but the core purchasers for the title will still be people who purchase anime and manga who are limited in the amount they can purchase simultaneously. Releasing double volumes in the same release pattern as now is likely to take up to much in the way of translation and QC resources, again affecting other releases.
The reason the novel will be difficult to obtain at the moment is, I expect, because Yen probably had a relatively initial small print run so they were not left with many copies if they didn't sell given how light novels have done in the past.
Oh no, I don't doubt a professional translator wouldnt have several projects, hence I mentioned doing two books at once might be more efficient, because he wouldnt have to adapt to a different writing style as often, in regards to resources then again double volumes would help here, as there would be less marketing, design work etc involved.

I don't disagree fan translations vary heavily, but that would be depedent on the people doing it, a translator of high enough skill to happily translate novels professionally is not likely to be a slow reader, I do get what you're saying regards quality though, it would really depend on the individuals involved, how much time they have and resources (a dedicated translator may take slightly longer to ensure everything is correct, but then that IS his job, most fan subbers I'd imagine will have work etc to compete with, so whereas the fan subber may only get several hours a week, the guy doing it for a job would have a far higher time budget).
The point on western novels will likely depend on what you read, I'm not saying there arent authors out there like the ones you mention, BUT the vast majority of western series are not that long, at least in one collection, so whilst you may get linked collections featuring the same characters etc, or the same universe, they're often not a continuation of one story over the a larger length, which spice and wolf undoubtedly is. I also read very fast, generally I'll get through a five hundred page novel in a day or less. We just have slightly different points of view on this sort of thing, I have some patience, but I get very into stories so in many cases I prefer to start a story part the way through so I can read several volumes at once.
I do understand your point on oversaturation, I've worked in retail and behind this sort of stuff before, and I've got family members who do planning jobs for certain large chains (it can be quite interesting sometimes), but I don't think reducing the time betwen volumes SLIGHTLY would hurt them, nor would a double book volume style release, as then they'd still be running on a 6 month rotation allowing room for thier other franchises (depedant on whether double volume translation was more time efficient or not, theoretically it would be, as it becomes easier to write about familiar characters/read familiar styles with time).
Without seeing any numbers it'd be hard to say one way or another on the limited run part, I agree you may be partially right I doubt the run was huge because of the experimental nature of the series, but then Spice and Wolf seems to have attracted somewhat of a following in America, as Werewolf said, he believes the DVD has sold pretty well there too, and a lot of the larger sites regular book sites seem to either have sold out, have limited stock of the title,or be waiting on new stock, which suggests there have been at least reasonable sales. The fact sites have been running out and having to order more will at least reflect well, and hopefuly mean Yen Press will be a little more optimistic for the series the next time round, if sales continue to be consistant, this may further persuade them the series may be worth speeding up the release of slightly.
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Beyond all rational debate though, I want the rest of the series yesterday, I know thats never going to happen though! haha.
I'm trying to hold off on reading my copy of the first volume, at least for a few days so I'm very slightly closer to the second on completion
Sorry if some of my 'argument' seems a bit threadbare right now, it made sense in my head but Im not sleeping well right now and have been running about 2 hours of sleep a night for the last few days so admittedly my logic might have more holes than I'd realised. Family antics are...fun ¬_¬