New housing schemes of this scale don't gain appoval overnight and there is a consultation period during which any interested party can write to object, and generally muster up lobbying of MPs and others for potential support.
You've mentioned 4 things here, access, environmental, ground stability and your vista across the hillside.
In turn: -
Access - this is definitely one the planners will look at carefully. It is not possible that the developer can make such an application without fully explaining how the scheme would impact existing broutes. Often this involves him commissioning traffic surveys. Get hold of the relevant part of the Planning Application and see what it says and see to what extent you can agree with it - then work out a counter statement.
Environmental - the EA would be involved in assessing the impact if water courses are being modified. Again see what has been said in the application and speak to the local planning department to assess who at the EA is dealing with it. Then approach them with any concerns they may not have thought of. They provide a report back to the Planners recommending (or not) the application.
Ground stability - unless you think it may impact your house, I'm not sure why this is anything to do with you. It will be assessed as part of the overall scheme. There are many ways of capping old shafts.
Your vista - one has no right to a view so don't even go there in your objection letter(s). Just makes one seem like a NIMBY. What is assessed in planning is overlooking a particular house or garden, but if this is across the valley it is not relevant.