That is sheer ignorance of the english language. It doesn't harm the quality of the novel at all if you know what you are doing. For reference, Shakespeare made up words for the convenience of his sonnets. Yes, he made up HUNDREDS of words! Are you going to also claim Shakespeare messed up the quality of his works to by making up words? Of course I'm not saying to just keep making up words for the sake of making them up, but yes you can make up your own words if it fits the need. And that is making up words. Tense switching is considered proper grammar, and like all things a tool an author can use if they want to or not.
Ya know, for one your comparison sucks harder than the average novel on NU. I also bothered asking multiple literature master students. Result? You can technically swap tenses. Is it practical? To quote the guys who read more english books in the last three years than you will in your life: "We have never seen it being used to any memorable extend. And that is assuming the author is exceptionally skilled. Usually its straight up terrible." What do we learn from that? It is a noob trap in the sense than any unexperienced (and with that i mean non-pro writer with tens of thousands of printed copies) will wreck the scene with it. You should change your name to professional shit-poster at some point.
Exactly. Making readers pay more attention is great if they're paying attention to the right things. But you can also make them pay more attention by pulling dumb stunts like purposefully misspelling every fourth word. Writing like this pulls readers out of the reading experience and is an absolutely terrible idea for anyone but the most experienced writers. That's because Shakespeare was an amazing writer who could make it stick. But just because Shakespeare could do something doesn't mean that an amateur can. Usain Bolt could run world record times on a diet of McNuggets, but that's insanity for a normal person to try! Guy Gavriel Kay swapped tenses in his books "Under Heaven" and "River of the Stars". But A. he is a great writer with excellent command of English, and B. he was doing so for a very specific reason. He would write chapters with male POVs in past tense and chapters with female POVs in present tense. This was meant to highlight how male and female characters in his pseudo-Chinese culture experience the world in fundamentally different ways. And even so, most book reviewers didn't bother mentioning it!