I remembered this thread because of the other thread about problems at school – I wanted to post about my experience in relation to the things you specifically mentioned here just in case it was helpful:
For me, his language ability goes when he is heightened. “What did you say?” Or completely wrong words eg fish for flowerpot (he did that when talking to a member of staff at nursery or elephants when he meant chair My dc who suffered from trauma suffered with this for a while. It is to do with the brain going offline because of stress/trauma/anxiety I think, similar to when adults lose their words, brain doesn't work so well when stressed, mind going blank during public speaking. I found that what worked really well was to get down on the same level and look into dc's eyes and hold their hand and tell them to find their words in their own time - they would then take a moment and find the word they wanted - the idea was to give them confidence that they would be able to find their words if they took a moment to calm down, and I guess over time they were able to do that on their own. This isn't a strategy I read about anywhere - it was more that I could relate as I occasionally had to give talks in seminars to hundreds of people as part of my work, and I knew that without the right props my mind would go blank, and i felt intuitively that if i supported my dc to find their words for a bit they would learn to do so on their own.
On my return he becomes excited (running on furniture, throwing toys etc) which one member of nursery staff commented it’s just not like him For this sort of thing (I know that your dc is at school now) I found that with my dc who suffered from trauma, he sometimes didn't pick up social cues because of the stress/brain being offline at the time of stress. I found that if I explained clearly expectations in advance this helped him a lot. I also found that when he knew what expectations were, he was less likely to flip into fight/flight though obviously not always the case. For your situation for example I would have said something along the lines of "when I come to pick you up, try to stay calm and stand or sit calmly, you aren't allowed to run around on furniture, but you can do xx" I have read on here that some adopters think that teaching social skills/social rules in this way is shaming or going against the paramount of "therapeutic" parenting and attachment, but I disagree, social skills and knowing social rules are important to fit in, and lots of children need to be taught them rather than just pick them up, not just adoptees. If dc are following expectations they will be accepted more easily, get more positive strokes from others, find relationships easier. In my experience it helps with attachment.
(another example from a friend)) to me this sounded "othering" because in my world it is really rare to find parents discussing their children with friends like this and so when it happens it feels sort of “othering” of the child – whether adopted or not. Others may entirely disagree with this, I don't know. My personal opinion is that it is best to only discuss this sort of thing with people who have expertise, who would be able to provide really good guidance.
Lastly, I think that dealing with dysregulated behaviour is usually easier for bio parents as there is that level of knowledge and relationship right from the outset with their child - whereas you are still getting to know your child - you may well have found with your first child, your birth child, that all this came easier and problems didn't become problems because you just knew intuitively how to handle things with them. That is why taking a step back and getting professional help may well turn things around for you.