As I’ve learned here, “irregardless”, no matter how silly-looking (and sounding), is actually a sort of acceptable form. Webster’s and American Heritage both accept it.
It’s worth noting, I suppose, that the ultimate authority - the good old OED - lists is as “informal”.
As do I. “Centred/centered on” would be correct if you’re talking about a physical location or something else that would be a discrete fixed spot - a number, for example.
If you’re using it to describe an idea or somesuch, though, “centered around” makes perfect sense; ie. “The book is centered around the concept of divinity.”
Since divinity is not an easily identifiable point, you couldn’t say something was centered on it, really.
Yes, but -able and -ible are commonly interchangeable suffixes and mean the same thing. “Comestible”, “edible”, “possible”, “eligible”, “dirigible” and others all use “-ible” in the same sense.
My personal gripe is a usage error rather than a spelling error: “within” as an inclusive term. It isn’t. Sorry.
If you have $5, and your goal is to have $6, you are not “within a dollar of your goal.” You are a dollar from your goal. You are within $1.01 of your goal.
Sports announcers do this all the time. The Bulls are down by six points. “The Bulls are within six with a minute to go!” No, you fucking idiot, they aren’t, unless they scored half a point somehow and the scoreboard doesn’t display fractions or decimals.