It's obvious that I'm NOT a big fan of H&M, but if that is so, I wouldn't call it immaturity, I'd call Harry not telling her about what to expect as absolutely deceptive. I'd put it on par with not mentioning that you have, oh, 8 children or so.
I consider myself to have a 'basic' education 'royal protocol' so I would have known about the curtsey, order of precedence + a bit more. BUT, and again I'm not a Meghan fan, we're also told that as Americans royal protocol doesn't apply to us. We can curtsey if we choose to, but it is not required nor expected. So as far as 'general ignorance', I might buy that it that someone might think the curtsey was 'optional'. BUT, she's not stupid nor naive and I don't believe for a moment that she thought it was optional in her situation. And I don't believe for one second she didn't 'know' who Harry was. Give me a break! She may not have known much about the RF but I don't believe she didn't Google extensively once they'd had a couple of dates.
Personally, I think Harry did 'lie by omission' and that shows someone who is completely selfish and without a lot of principles. I think he's spoilt and used to getting his own way. But I also think Meghan's "I didn't know" about who Harry was, about the RF in general, or about protocol is pure bullshit. She knew. I just think she thought "Well, I won't be doing that, they're going to have to deal with me as I am". They both thought 'our way or the highway' only to find out that 'The Institution' had all the power.
I think they both have a bad case of 'sour grapes' and are throwing the nastiest tantrum that's been seen in a long long time. Even Edward & Wallis behaved with more dignity than those two. I feel for Archie & Lili when they get older and read about all this crap. This isn't the 1960s where you only learnt what your parents told you about 'family squabbles'. This is all going to be 'out there' for them to see on the internet.