I agree there's little point in correcting minor typos/ inaccuracies - things like saying you were driving a Fiat Panda when it was a Fiat Uno, I'd just point that out when disclosing the report to insurers. A more serious issue, like the expert hasn't recorded info you had disclosed, like details of a previous accident/ injury, treatment you'd already received or ongoing limitations...that is the kind of thing insurers will pick up on if it is contradicted elsewhere and that is something I'd want addressed.
I can't speak for all insurers but working with one of the biggest, they absolutely would make an offer based on that first medical report. It would be a generous offer (ie more than the value of the injuries based solely on that first report) to take account of the fact the medical prognosis isn't complete.
As to how much it would be...I can't say because I don't know what information insurers have. They may take into account the loss of your business; alternatively they may feel that cannot be directly connected to the accident. They may feel the content of your medical records shows you are not experiencing many ongoing effects (so their offer will be lower) or the opposite - which would result in a higher offer.
What stands out for me though is you really aren't happy with your current solicitor. From what you say about the lack of contact and delay I can completely understand this. Is it a large firm? If so I wonder whether you could ask for the case to be transferred - that's what we would do if there was a complaint; I know because I used to get a lot of the 'complainers' (most of whom were fine really and just frustrated at delay or lack of explanation from more junior staff). If it's only a small firm, I would consider instructing someone else. I know you are keen to get matters moved on but you might find that even with the inevitable delay involved in another firm taking the case over, on the long run it is quicker.
We do see a lot of simple cases with far more minor injuries than yours (straightforward whiplash for example) taking far too long to resolve, upwards of 4-5 years for some. Part of that seems to be lack of instructions from the client, but often it will also be delay on the part of the solicitors firm. Those unnecessary delays frustrate insurers because they prefer to settle claims sooner rather than later.