index.php doesn't point to website

Hi,
I know this isn’t the right place to post but I couldn’t log to the WP-Codex forum!! If a moderator sees this, please put me in the right area.
Here it is:
I’m using a mac, FTP is with Transmit. I am doing a website for a client on WordPress. The site is finished but my client wanted to add another page in front of the website so when you click on the URL in Google, you have a page with their logo which you click and then access to the wordpress website.

So : I created a ‘index.html’ with dreamweaver pointing to “https://www….” and I uploaded it in the FTP. The ‘index.php’ of the wordpress site points also to that same address.
I added this code here in the ‘.htaccess’ file:

‘DirectoryIndex index.html index.php’

I then had to replace the ‘index.html’ by another one (same but the content image is just bigger to have full width) and removing the old one. And now nothing seems to point, or there is a duplicate problem! I have the “Internal Server Problem, error 500”.

So now all I want is having it back the way it was,just the website I did. I deleted the ‘index.html’ file and wrote in the ‘.htaccess’ the way it was before making changes to it. Of course, this didn’t work either so that’s my question all the way. Help!
And I can’t seem to access the “error.log” from apache either – so if you have tips on that too !