Howdy jddvette,
I think this is your past ticket: https://theeventscalendar.com/support/forums/topic/default-categories-to-list-view-previous-link-is-not-working/ I went through that and tried to duplicate the issue. The snippet you linked forces list view for category pages. I put the snippet you linked in the default theme’s functions.php, and indeed found that only list view was available while viewing a category.
In that topic you said the Next/Prev links were not working on the Event Details page. However, in my test setup those links are working fine. In this topic you stated that the Next/Prev links are not working for month views. Again in my test setup those links also are working fine.
You seem to be the only one experiencing this problem. It is doubtful that our next release will do much to fix it.
Without being able to see this in action and use browser diagnostic tools our hands are a bit tied. It could be an Ajax issue, but it also might be something else. If you are convinced it is an Ajax issue, commonly those stem from not having CORS (http://en.wikipedia.org/wiki/Cross-origin_resource_sharing) properly configured on your server. Are all resources on your site on the same domain and subdomain?
Have you had any progress with your client in getting the internal servers opened up so we could look at it? I would like to help you fix this, especially before your client notices. But it will likely require access to site. Right now any suggestion is a stab in the dark.
– Brook