Your mistake was appointing a solicitor suggested by the builder.
You can clearly see by their response that they are leaning towards the builder.
I deal with pre purchase surveys etc regularly and to be honest, I would expect your solicitor to strongly represent your side and claim initially at least in writing that the works are not as agreed / as per site layout drawings and would need to be rectified or deem the contract void and return deposit.
Whether there was a leg to stand on would be teased out after that but for your solicitor to straight up tell you take it or loose deposit is scandalous in my opinion.
The paving can be completed where manholes are needed. There are recessed manhole covers that allow paving stones sit into the cover and look very well.
If they don't play ball, have an Engineer/surveyor on your behalf (and not one suggested by the builder) inspect the finished works for planning and building regulation compliance.
If would not surprise me if thousands of euro worth of remedial works could be found by thorough surveyor.