I understand that, Wendy, but then we run into issues like when I bought my CA (before I was a tech) and I'd been told on this and other forums that it was OK to ignore the level line and it would be a great fit in my Civic, because some customer service rep told someone once that it would be OK more upright and that the level line was only a "suggestion" and was intended to be the newborn angle only... then Dorel comes out with a bunch of cryptic statements implying that your car was going to melt into a puddle and burn your child to a crisp if the level line wasn't followed (oh, but when pressed about it, they admitted it passes crash tests more upright, but still wouldn't change their advice). I never would have bought that seat if I had known I'd need to ignore the manufacturer's instructions/warnings to make it fit in my car. There are plenty of other great choices out there with reasonable instructions I could have bought instead.
As a parent with a 10 month old, might I use an angle adjuster in some circumstances? Maybe. As a tech, am I going to advise a parent to use one against the written instructions without suggesting the parent contact Diono directly and asking? Not unless absolutely necessary, and I'd try to avoid it in writing in a forum that might lead someone else to say, "My 10 month old was fine," and then "My 8 month old was fine" and then "My 6 month old was fine" and then before you know it you have someone putting a Radian with an angle adjuster at a 30 degree angle for a newborn on the way home from the hospital.
The company had to draw the line somewhere. Is the line a little blurry? Yeah, but it's still a line. Is the OP "wrong" for making the choice to boost up a very reclined Radian for a 10 month old with good head control? Probably not. I'm not going to try to convince HadleysMom to change her mind like I would if she were determined to turn the Radian FF at this age.
But I just think we need to be more careful before we pass out "It's only a suggestion" advice all willy nilly.