Hi GertyD - Hope it goes well meeting him tomorrow!
I think you are probably right about your SW needing to meet targets, and she will be very nice to you to get you on board. We found out in retrospect the SWs contacted us only a month before a judge-imposed deadline for finding a permanent solution for our DNs. No wonder the SW we met was very breezy & optimistic, "these children just need some good parenting" etc.
I think it's really positive that your DN would be able to continue at his school so he has continuity of friends & support. Its taken us ages to get all that in place.
Re the behaviour: its not necessarily aggressive behaviour towards your 2yo, it more juggling the competing immediate needs / demands (which may manifest in different ways over the years). My two, though 4 and 5, act like toddlers with attention-seeking & risky behaviours, & don't like it if I pay any attention to my older son, try to use the phone, go to the toilet etc. We have to keep working to make sure my son does not feel excluded (so at the moment lots of late night watching world cup football over takeaway pizza with his dad). Its great your boy is already with CAMHS so you won't have to wait for a referral.
Remember the SW is acting for the child, not you, so you have to look after yourselves, get informed, keep a good diary & notes (follow up meetings/phone calls with emails so you have a paper trail; you will deal with many people from the LA, school, GP, CAMHS, court etc and SWs have a high turnover). Read the adoption boards / literature early so you know what you might experience (cos a SW against a tight time target won't tell you any of that).
Contact the FRG to find out about what support you may get from the LA, especially if you go the SGO route (we found the SWs inexperienced in SGOs). For example the LA should pay for you to have independent legal advice; possibly a means-tested allowance, a settling-in grant and other expenses (we got a small amount towards removal costs of moving to a bigger house). All given grudgingly and after months of wrangling.
Family members: yes, lots promised to support us when they thought the other option was adoption. Now they only really contact us to ask to see the boys, which can be very disruptive (more unsettled behaviour), so take offers of support with a pinch of salt. Or pin them down to a particular task. For instance I have asked 2 family members to accompany me on contact visits with the birth parents and, I hope, help to organise them in the future.
Finally, I have to say this to you in case no-one else does. You are not his only chance. If he did not come to you he would be with a foster carer / family who would be lovely and very experienced in helping him through his early experiences (if you are not sure about that read MN's foster and and adoption boards, they are full of wonderful FCs). In these situations where the authorities are pushing for a quick resolution and family members don't want to "lose" a child there is a huge feeling of jeopardy and very heightened stakes. However you have to think very carefully about what is right for your existing family - your DH and DCs. Try and minimise the feeling of "this is the right thing to do" as that won't be very helpful when times are tough. You don't want to end up feeling that you are mopping up other people's problems (if you see what I mean).
Let us know how it goes tomorrow!