It may have been cheaper than Coulport, but that isn't just a special weapons site, it's conventional as well. So at the time, US basing might have been cheaper, but wouldn't satisfy a duel role. Of course other ordnance sites could have been expanded, but Portsmouth for example now wouldn't be practical due to other development reducing the sites size.
The conventional side of Coulport would also need moving, along with the non V class boats, Astute etc as the entire Submarine force is now planned to be based up there.
On replacement, it depends on what that replacement is. If air launched, its far less problematic, there are plenty of RAF Stations that retain an SSA, albeit unused, reactivating those is a cheap option, as is QRA 'arsenal' aircraft using either tomahawks or storm shadows with nuclear warheads. Not an ideal solution, but cheaper and the infrastructure while not is use exists.
In any case while disarmament might be the SNP's aim, is it really something a majority of Scots actually support? If they are ousted after independence, will an incoming Government follow that path? The other question is from the UK's point of view, which is would we be happy having our nuclear base in a foreign powers territory? when that power could in theory at least prevent it from sailing?