a sem-d means semi-detached which means that house must be attached to another house on one side. if you do not want to have your house attached to the other side (become a detached house), you can do so BUT there are many conditions to fulfill. assuming that your site is able to fulfill all conditions, the house you want to rebuilt must have the relevant setbacks of at least 2m at each side and the rear and the standard 7.5m at front. also, the site coverage for a detached house is limited to 50%. eg: if your land is 400sqm is size, the footprint of your house can only be up to 200sqm. this limiting factor is what cause some owners who own 2 side by side semi-d units to still retain the houses as semi-d instead of a detached house as they can have more built up areas. not to mention that you will most probably need to pay development charge to URA as you are "upgrading" the house from a semi-d to a detached which increases the value of the land.
of cos you still can build your pool on the side with the party wall but this will mean your house will be inside your house instead.
for your green buffer issue with NParks, your agent can write in to ask. but it is unlikely NParks will give you a clear cut answer now as they may want to see the entire development plan from a QP first before the commit to anything. There are several agencies to approve any building plans and NParks is one of the agencies who need to approve.