Hi Ydop,
You might be able to get away with a modal pop up without AdWords noticing, but I think that it does violate Google's policy in spirit, since Google says "Google does not allow pop ups of any kind when entering or leaving your landing page." (Bolding for emphasis) Even though they're talking about separate windows, the experience is the same for the user. In fact, sometimes it's more frustrating, since modal pop ups are often harder to close.
That said, your intent is fine, I just think that you need to use a different solution to introduce your company on your inventory page. You said that you want to use a pop up to tell AdWords visitors that your inventory page is full of hand-crafted furniture, but why can't you include that message at the top of the page? Especially if it's a core value of your business, visitors who started on the homepage won't mind being reminded about the quality of your work. And it will help visitors who land on that inventory page through search or referral links as well as AdWords visitors.
As a consultant, I push my clients to make every page include enough information that it can be a landing page. With Google trying to place visitors on the most specifically relevant page possible, and referring links out of your control, you never know where visitors will start on your website. Don't assume that visitors will start on your homepage, or that will begin their journey through your site understanding your business.
Good luck!
Kristina