Can it fit a 12.9 inch iPad Pro?

With the medium bag it’s a right fit. I’ll be ordering the large bag.

Is this travel ani-theft grade? Meaning it can not be sliced/cut?

I don't think that for the price that people should expect it to have materials that cannot be sliced. If you are looking for that high grade, should be looking at Pacsafe or the likes.

With regarding to those 1-star reviews, how do you guarantee that I can get a decent backpack?

Thank you for your question. We are glad that you are interested in our products. Our backpacks are specially designed by a team of professional designers, handmade with our craftsmanship by every meticulous stitch, focusing on every detail of the processing and striving for perfection of the product, Which shows our responsibility for the enjoyment of the dear customers. So we can confidently recommend the most cost-effective products with good material, attentive design, and first-class workmanship to everyone and bring you an enhanced product experience. Hundreds of thousands of buyers had bought our backpacks until now. When you see 97% positive reviews which depends on the support of our customers, and it means you have a 97% chance to get a good product. But we don't think that is enough, and we still need to work hard and try to achieve 100% satisfaction. For any product with any quality issue, we offer a 100% money refund or resend a new one and we afford the shipping cost. You have ZERO percent of risk to buy from PINCNEL!