Hi Daniel, I am very glad to see you've sold several thousand copies of your novels. You have some great story ideas, but I do want to add my thoughts to your points:
Point One: Truth that December is the best month for selling, period, where people buy gifts. Though if you build enough of an e-mail list you could create your own day or week of the year. This way you can brand your own day and make sure parents remember August 24 is "back to school with the Kenney Gang" or something like that. And then keep doing it every year until you see momentum from your fans as they begin to remember your chosen date. Something I'd like to try. Same with seasonal selling.
Point three: good point. It's almost impossible to figure out why some books sell better than others. If publishers knew this, I have a feeling almost every book they'd publish would cover the advance. Out of curiosity, which of yours are selling well?
Point Five: I wish you had talked to see sooner. I actually had a survey completed with book readership information (check out my blog samthefriedmanblog.com) next Tuesday and I'll post the survey results). Neilsen Bookscan did their own poll and found Middle Grade is the hardest to self-publish because it's the only genre where the purchaser is not the primary reader. Kids usually ask their parents for a book, which means whatever's popular in school. Indies with great reach in schools can pull this off. Most indies, however, do not have the name or marketing power to get lots of kids reading your book.
Yes, there is a difference between books for 8-10 and 11-13. Mine is Upper Middle Grade, ideally for 10-13.
As for the length, this is the biggest controversy with authors of all genres. For example, my first novel in my series is 73,000 words, or 5,000 less than the first Harry Potter. Publishers generally say that's too long, and at about 280 formatted book pages, this would be controversial. But...many of the bestselling books (HP, Fablehaven, Hunger Games) with tweens are actually longer than the average middle grade 130-150 story. Limited data I've had from kids suggests the length doesn't matter, they read it in 1-2 days. I am a sincere believer that it's the interest of the story, not the length, which ultimately drives any reader's interest. They will devour a great 300 page story but toss aside a 120 page one if the 300 page is deemed "better".
Point Six: yep, the reality of a writer

. I think you're one of the more successful middle grade indies though, and a testament to hard work and some luck.
Point Seven: Another contentious point.Some authors think you should publish at least x number a year, other say better to write the highest quality possible story and hope it sticks. As long as you are truly proud of every book you release, it doesn't matter if you publish 3 a year, 9 a year, or some other number. In any case, best of luck.