There is plenty of Room at Cornbrook. The platforms wouldn't have to perrfectly align with the existing metrolink platforms. They could be offset to the North East. Two tiny bridges over the entrance to the Trentham Street Scrap yard could give you platform length all the way to the Bridgewater canal. Then use one of the disused railway arches as your underpass.
Out side of the railway environment, you could build it for peanuts. Even adding in the railway tax, I reckon it would be a fraction the cost of Maghull North. I do not think this is the solution to platform 15/16 at Piccadilly, but it is a great solution for shutting Deansgate Heavy Rail, and allowing a decent improvement Castlefield junction to be complete without loosing *'precious' Heavy Rail - Metrolink connectivity.
* I say 'precious' because no-one ever seems to consider inter-modal connectivity as sacricant in other geographical locations within the UK.