How do I set the node path with a contextual filter?
First of all you don't need Contextual filter in your first case where you are generating link as "node/[nid]" , what you need is replacement pattern. Todo this you can simply add nid to fields list and place above read more link and use the pattern.
Change read more link to "jobs/current/posting/[nid]"


Regarding what you are trying to solve


You cannot have a view with path "node/%" since it's already taken by node module to render node pages. It will conflict with node module and as you said "bombs out ALL of my other content"
In your view set path as 'jobs/current/posting/%'
Add a contextual filter nid and set provide default value to content id from URL as shown below.