portfolio parallax responsiveness on mobile device

I have a couple of remaining issues for my new site on mobile in terms of the portfolio parallax - the site is www.guided-mountain-walks.com
There is a huge gap between the bottom row of images and the bottom of the parallax. This is a problem on smartphone when using desktop view and for either option on tablet (narrow/portrait screen).
When I remove the desktop view on smartphone, I have a different problem - the 2 row of images seem to have changed into one column with the only the top two images visible. Again, I would very much appreciate your help in this,

If you are looking at this, may I add one final issue which relates to site responsiveness on mobile devices, so I’ll include it here. On both smartphone and tablet the dropdown navbar links are not responding on all parent pages. In the case of this site, this affects the Welcome, Destinations and Terms & Booking page. The problem resolves itself when I change to desktop view. These are the final two issues which I hope to resolve so I’d appreciate your help.

Wondering if anyone can help me with this - as I said these are the remaining issues which I need to resolve before I can launch the site properly so any response would be appreciated. Thank-you