The meta description tag doesn't influence SERP's.

As you know search engines often use it as the description for your page in search results.As such a relevant, unique, compelling meta description can have a positive influence on how many clicks you get on your search listing.

As well as pagination pages I have read, but not done it myself, that you should consider using it on low value pages that you are wouldn't want to rank above other pages on the site (hopefully they wouldn't anyway) and also sitemaps as don't necessarily want them to appear in the index but definitely want them followed.

As you have looked at the traffic sources and they seem to be natural I would then look at what the visitors are doing on your site.

Is the bounce rate up considerably? Has the time per visit fallen dramatically?

If not, if the traffic seems to be coming from a natural spread of sources and the behaviour on the site of the visitors is normal then just be glad of the extra traffic.

The way to rank for spelling errors is to optimise for them. Rather than littering the site with poor spelling which will not look good to the user you should try to include the misspelling as genuine content.

For example a light hearted blog post with incorrect spelling vs correct spelling in the title and page name. In the content t could mention the spelling mistakes and unusual terms that people use to find your site (gathered from the analytics data).

It may not be the most interesting post for the visitor and will struggle to gain page specific links but you will have a page optimised in terms of content for the misspelling. If you don't really want people to read it release it to the site at a quiet time and then follow up with a stong post which people will read and the spelling blog post will be overshadowed.

Hope this helps.

Depending on your analytics package you should be able to look at information such as network and location of visitors and drill down into them to see if the traffic is coming from an unusual source. For example on a UK site I look after with a small local focus they have had a number of visits from Tawain via the Taiwanese Google site. This may give you some insights as to whether it seems to be a real bounce or something more sinister.