SWR won't have bought them, they will probably have leased them to the end of the current franchise. If the government changed the spec in the meantime to add toilets then SWR would want compensation. If they specified toilets in the next franchise then the new franchisee would have to provide them and would include the cost in the franchise finances, so ultimately the taxpayer pays in reduced premium or increased subsidy. The other option is that the lease is protected so that the future franchisee has to take them on, when again the government would be liable for the cost of adding toilets if they were to specify them.
Given the modular design, Vivarail could most likely add toilets later to units that didn't have them added during conversion.
These trains don't even have air suspension that could be deflated. But the other Vivarail units have been shimmed to lift the bodies up a bit from the original D78, so they clear standard platforms. As the platforms on the IoW are low to suit the existing stock, there is no need to do that. I think they mentioned level boarding, so these units will probably have the bodies as low on the bogies as they can get them, with the platforms raised to match that height.