I am interested in buying the game, but after playing the demo,and reading many reviews, the consensus is that it's extremely difficult to trigger event or complete routes. Can the developer provide a guide or sell it? I feel hesitant to purchase the game without a walkthrough.

I suggest you buy and play without a guide sometimes, with at least one of the guys you can get some good cgs right away. But all the others are really tricky without a guide. 

The dev is selling a walkthrough but very expensive, I don't recommend buying it. Apart from that even here in the comments you have some tips on how to respond on some routes. I think the fun of the game is that it's not that easy. But don't expect much development in the relationship with the characters, everything is very superficial. But I've played worse.  Also, don't try too hard with David if you're interested, his route is the most complicated and the walkthrough is the most expensive sold by the dev.