main menu stacked vertical instead of horizontal

Try deactivating plugins to explore whether any could be responsible, you have more JavaScript errors on the page that menu is broken.

Looks like that menu is in a table – and the width is constrained here:

table, td, th {
    border: 25px solid black;
    width: 275px;
}

The main menu in your site does not appear “stacked”. When I clicked the links both main menus were horizontal. My guess is maybe wordpress consolidated it in your menu so that you can see all of your edit options.