ZC 1.5.1 store is access completely restricted to logged in customers only.

I have created a external landing page which looks just like my cart pages in the document root of my website where all traffic to "www.somedomain.com" goes. My store is in a subfolder callled "somestore" (not really).

When I access the external landing page and click the "Log In" link" I have created in the header h t t p s://www.somedomain.com/somestore/index.php?main_page=login" it goes to the log in screen of ZC.
So far so good..... (Customer decides not to login)

Now when I click on the "Home" link in the header instead of staying at the cart log in page (like it's supposed to) I would like it to go back to the external landing page.
What I want is to have the "Home" link switch between logged in (normal operation) and not logged in (Go back to external landing page).
This probably means adding some code to distinguish between a logged/not logged in.
Yes, this may be a core file edit which I'm willing to do......

Maybe there is already a solution for this but I have seen several persons asking for similar functionality but haven't been able to good solution.