The second question here is rather easier to answer than the first. The PM can give an instruction that changes any part of the Scope/Works Info so that may lead to a change occurring off site. For example, where design is taking place off site of course the PM can instruct a change on or impacting that design.
The difficult area and cross over with the first question is whether the PM can, on its own, instruct work to be carried out off site and if such an instruction is given do those works fall under the contract. Again, the second part is rather easier, if a lawful instruction is given it must bring with it all of the terms of the underlying contract either because the PM is only a creature of the contract and can only work within its boundaries or, if the PM stepped beyond that, it would be acting a agent for the Client (probably) and therefore the starting assumption would be the same contract is intended.
That brings you back to the difficult question of instructing works off site. In my view this depends on the type of work. If the contractor can lawfully carry out the work then the change in Scope is acceptable whether on site or off site is possible. So, as mentioned above, a change or addition of off site design is fine. Coming back to the specific question here the PM has no power to change the boundary of the and the Contractor has no right to carry our work for the client off site (there is a licence for the same given by the Client for the site). If the underlying instruction cannot be given then a change cannot be given either. So the “agreement” to use the off site area shouldn’t have been given as the contractor had no right to work there.
There is one saving I can see. If the off site area is also owned by Client then the PM, acting as agent for the Client, may have extended the site. As a matter of practicality that seems to be what has happened (no doubt for sensible practical reasons) but getting the legal interpretation to catch up will be a little tricky and require some care. The point remains however that if the work is lawful (ie a right to carry out the off site work can be demonstrated) then the identification and change to deal with asbestos found MUST follow the same principles as the underlying contract.