The only way I know to do this is to use a 3rd party tool like DocAve
Content Manager. (The DocAve backup/restore solution does not allow you to
promote subsites to site  collections).

I've done it and it's really easy with DocAve. They have a 30 day trial too.

I could accomplish  it using stsadm export and import.

Stsadm export and import will not turn a subsite into a root  site. Although
you can use it to move  a site  to a different place in your URL structure, it
will not take on the characteristics of a root site.

This is what I did-

Exported (stsadm export) subweb- created new site  collection (siteColA) with
blank template, imported subweb to new url.
Exported (stsadm export) the root  site (siteColOLD) collection- created a subweb
using blank template under SiteColA- imported siteColOLD under the new subweb.

Will the above work? It seems to work right now.

Since what you are doing is in effect merging it into the URL I suppose it
might, to a point. I don’t see at all where this has any bearing on the 100s
of other sites you mentioned, all of which are now in a totally different
Site Collection. There is no connection between site  Collections for things
such as navigation, Content Types, Columns etc.

Also, be aware that the import/export commands are not full fidelity copies.
You don’t get user personalization’s, alerts and such.