Agree with everyone else. Long before you got to that bit in your OP, I was replying in my head that neither the SaLT or the HV have observed him over time, in a Nursery environment, so how can they pass judgement on his behaviour after a short time in what - to him - is an alien environment ?
The Nursery are obliged to do assessments of all 2yr olds - that is done for every child, not something they are doing just because they are worried about some of your ds's behaviours.
It's surely good that they are inviting in their Area SENCo / Inclusion Officer for further advice, if they have concerns.
The staff at Nursery work with dc your son's age, day in, day out, year after year. If they are saying some things he does are unusual, then I'd listen to them. No, they can't diagnose anything, but they have to mention concerns to you and they are supposed to suggest referrals for deeper assessments if they feel that is the best course of action.
I'm not sure asking you how long the SaLT or HV spent with him, is "getting aggressive" but obv I wasn't there. However, it's much easier for them to just plod along and not raise concerns with parents - why on earth would they want to "cause anyone hassle"?
. They are doing their job properly, by raising concerns with you.
What I would do is ask them if the person has been in, and what she suggested / advised, then try to work out a plan together of strategies they could use to try to help him become more settled at Nursery.