my son was really very similar to yours at school nursery. they flagged concerns about attention, refusal to comply, melting down when routine changed. fine one to one with an adult, chatty, etc, but no social behaviour with other children. also a mild speech delay and a drawn-out way of speech, he had a very "baby" sounding voice till about 6yrs old. he also refused to do anything unless it was something he wanted to do. imaginative play etc at home seemed normal and he adored - adores (also dominates) his younger brother (2yrs younger). didn't have a clue what parties were about. would never ever do circle time or stories or singing at any toddler group I was at with him and they said he would not join in at nursery.
so they flagged this up, I was upset, they told me they didn't have to take anything further/formally, and that the likely suggested approach was what they would take with him anyway. I wanted to put all this down to him being an August boy who'd never been in any childcare setting and he just wasn't familiar with it.
fast forward to reception - which was mostly ok, some behaviour problems but not really anything that made them want to raise a concern; ditto year 1, though he was not progressing academically; they used to comment that he would seem to be in his own world when the children were on the carpet for teaching but when addressed directly he would usually be able to answer a question, which indicated he had actually been listening after all. by year 2, it was very very apparant how far he was starting to diverge from the rest of the children socially, and that along with really clear attention and focus problems raised by his teachers (he was in intervention math and literacy because of the focus problems) and my own experience with him being harder and harder (at times impossible) to manage at home when it involved doing something he didn't want to do like going to school, bed and homework. after meetings with the senco, headteacher and his class teachers, we agreed to set the assessment process in motion.
from initial GP appointment to the assessment, it was about a year, and after going through the parent part of the assessment I was in no doubt that the psychologist was going to say he has ASD. my son was 7 and a half. SO much stuff I had no realised were indicative of autism spectrum from infancy and toddler years... and it was very clear as in my head I was considering all the many many times I had had the flash of realisation that DS1 was not like this but DS2 was. or vice versa. I almost icked myself for having any sense of surprise at all at the diagnosis. of COURSE. it all makes such sense.
hindsight being 20/20 and me having not wanted to see a lot of it through fear.
if your child has ASD he has ASD. if not, then he doesn't. I don't see the harm in seeing what a profession assessor says. it can only help you support and understand. I really wish I'd pushed for my son's assessment a little earlier, purely for the sense of acceptance and understanding for me and at school, and for him eventually, too. it's hard to watch him struggle with friends (he wants to force his friends to do as he says and doesn't cope when they won't).
as for the ASD characteristics that my son, when 3, did not have, at 7yrs old he certainly does. obsessions/special intersts? check. hand flapping? check. meltdowns? check check check. he showed no signs of any of that till he was 6. frustrated tantrumsm sure, but they seemed age/developmentally appropriate.
oh, and my son's lingering tendency to elongate words was noted by the psychologist who assessed him, and SALT has been recommended, alongside assessment by an educational psychologist. the main assessor said he did not consider he has ADHD, but that the inattention/focus stuff was all part of the ASD.