At risk of sounding like an armchair psychiatrist, I think greeting people who might not return the same enthusiasm shows inability to read intent and could fall under the social deficit criteria in autism.
I also think that having "childish" or unusual interests, especially if they're intense could fall under the restrictive and repetitive criteria too.
I agree with other PPs that it also sounds like she could have Williams syndrome, but people with Williams syndrome usually have some distinct and unique facial features. There are also usually co-occuring cardiovascular issues, developmental delays and connective tissue abnormalities, which you've not mentioned any of these so it seems less likely to be Williams.
For many of us autistic women, diagnoses don't come until later in life when the deficits in communication, understanding intent, displaying naivety, and not being able to recognise emotions really sticks out as the supports in place from childhood are no longer there. There are no longer teachers supporting friendship groups or encouraging cooperation, or safeguarding against others with poor intent. We often assume people have the best intentions.
We don't always have sensory sensitivities, in fact, sensory sensitivities alone aren't part of the diagnostic criteria. They can play a part in restrictive and repetitive interests/movements/rigidity, but being hyper/hyposensitive to sensory input isn't necessarily part of the criteria.
It's also overlooked in a lot of women especially as the stereotypical social and communication challenges function around low register, less likely to engage especially if something is not of interest, not being able to make friends or form friendships, and often overlooks the challenges that come with being hyperverbal and extremely sociable even though there are clearly deficits if she can't read someones intentions, mood, energy levels, and forces herself into conversation with people.
I worked for Starbucks for some time, doing an apprenticeship, and I was very much hyperverbal, pushing myself into conversation, and I made for a great sales person, because I could push their coffee of the week, rewards cards systems, additional extras primarily because I couldn't see the disinterest that people had, and my workplace used this to their advantage. I was young, barely 18, and I found it such a strength, but then as I moved on from that workplace into other settings, and found myself having to form my own social groups and not finding a community that had been manufactured by schools or employers, it became really apparent that what I was doing was more of a weakness than a strength, and was considered abnormal and restrictive. I'll spare you the long story, but it lead to a lot of sticky situations with male friends and not being able to pick up on their social cues, and lead to some rather intimate actions that I felt I couldn't advocate for myself for or get out of because I had "led them on". Me thinking I was being friendly, and them thinking I was being friendlier. So I do think that you need to consider the wider safeguarding ramifications given the deficits you've listed.
I think the average age for diagnosing women is 36-37, and it's that high for a reason. A good psychiatrist will be able to ask you the right questions to confirm that these struggles were in place as a child just less evident as she had the right support structures around her at the time. They may also ask her to take part in a CATQ screening for masking.
Having childish interests and hobbies in itself is not an issue, but it is restrictive in the fact that she will struggle to form relationships with peers appropriate to her age group, and that's how they would view it in an assessment setting.
She is almost an adult, so I would do some research and see if she would consider an assessment, but I think your immediate worries need to be around safeguarding her and preparing her for adult life, and most importantly helping her understand consent and cultivating a safe space with you to talk about any issues that may arise due to her communication deficits.