First of all, as a simple metric, your use of the word 'that' is 50% higher than my own (I have 930 occurrences in 100K words.) I've tried to minimize 'that' in my writing. It's one of the words that can be pruned out in revisions without losing meaning (like the word 'just')--and as a side benefit the pacing of the writing is often better.
And, as another note--there are questions on this SE that look at the values and pitfalls of writing groups. Basically, writers' groups come with pros and cons. To my knowledge, there are no questions that ask whether the feedback from writers' groups is generally more or less reliable than that from publishers.
But to answer your question, here are a few points to consider:
Authorhouse sells a service. This calls their feedback to you into (serious) question. As a service provider, they have zero reason to tell you that your work could be better.
Writers' groups are variable, and yes, they focus on finding the weak spots rather than the strengths. Go to a different writers' group and you will have a different set of responses. Some responses may overlap, and this will start to point to a pattern in your writing.
Some individuals in these groups tend to focus on the 'thing' they are currently mastering, such as echo words or character voice. A woman in my group is currently focused on whether she feels the emotion or not, in my excerpts. This is her feedback, routinely, because she is focused on emotion in her work at the moment. The reason this is valuable is because she will pinpoint areas where I might actually be able to increase emotion--areas I'm blind to, and she spots them like she was born to do it. The feedback you've received may be considered along these same lines.
Some writers' groups coalesce around a certain style of writing. Thrillers, or cinematic, or literary, or women's, for example. You can either find the group that matches the story you are telling, or find a group with a more diverse set of participants, or learn to weight feedback accordingly.
Every single person's feedback is unique. Broadly speaking, as groups, family often provides kind feedback, fellow writers are more often technical and craft-focused, service providers want your business, and non-writer readers (especially those whom you do not know) will be all over the board in regards to how they read your story. They may not know the difference between first and third person narrative, for example, or have a grasp on viewpoint.
Good advice is to take any critique that resonates with you and allow the rest to fall by the wayside.
More good advice is to find a reliable set of critique partners that you trust, and whose feedback is helpful, for the long haul. Finding these people takes a while. Through writers' groups is one way to go about it.
More good advice is to take the critiques you get, let them sit for a while, come back to them some days later and see if they makes sense once the initial sting of them has eased.
More good advice is to read craft books, and books in your genre, and ask yourself how your favorite authors accomplish certain effects, and whether you are accomplishing the same through your own style. Certainly some authors repeat words within paragraphs although I don't recall seeing a memorable word like 'unremarkable' four times within a paragraph. I'll occasionally repeat a word, or a paragraph style, intentionally and for effect but I trust my critique partners to tell me if it works for them. (ex: She hated the look on his face. She hated his piggy little eyes screwed up in judgment. She hated the pink flush on his cheeks, and she hated the way his chin quivered into itself whenever he spoke to her. She hated his sweaty hair, his sweaty forehead, and his ridiculously sweaty neck. But most of all, she hated that he made her feel completely and absolutely inadequate to the task.)
That's obviously repetitive, to make a point. It might work or there might be a better way to convey her feeling in this moment. If you are repeating the word 'unremarkable' to make a point, it's different than if you are repeating it unawares. A writer should be aware of their writing, and every sentence should have a purpose and be written intentionally.
As a final and unsolicited thought, 300,000 words sounds to me like a very long book, and it sounds as thought this might be a debut for you? If you are self-publishing, first of all I congratulate you, but second of all I do wonder if you can shorten it downward to your benefit. 300,000 words is a lot of pages, and this translates into cost to print, and a potential customer might decide to pick up a 100,000 word book instead because the print cost is more manageable for their pocketbook. You'll notice that many hefty works of fiction are not an author's first book, but instead come out after the fan base is established.
Edit: I forgot to say something about character voice until I read, enjoyed, and +1'd the other responses.
Character voice can be made distinct in many ways. Through dialog, action tags, adjectives/adverbs, narration. Try (1) a dialect, (2) different euphemisms, (3) quirks of word usage.
Try using the occasional (4) adverb or (5) said book-ism or (6) description of style of speech, limited to a single character at a time, to make their normal dialog come across as given in a particular way.
ex: She quipped, or she said with the snark she was known for, etc.
(7) I've seen authors characterize speech through observation. Gatsby says that Daisy's voice is 'full of money.' It colors how the reader 'hears' Daisy.
Try (8) using action tags like She waved her hand airily as she spoke to convey a certain voice.
Reading with an eye to this sort of thing opens up possibilities, and there are craft books that get into every nuance of writing, too.
Have fun with it.