The Metrocar roofs are lower than standard trains because on most of the network the overhead line was put under existing bridges so the BR standard clearance no longer exists. Exceptions are from Benton to Fawdon which was used by BR freight trains, plus Pelaw to Sunderland of course (not sure about South Hylton).
Normal tram maximum width is 2.65m, as in Manchester, Croydon, Blackpool, Sheffield, Edinburgh. Nottingham is narrower due to tight curves in the city centre. Metrocars are wider than this, as they reach almost to the platforms even on those routes where heavy rail did or does run. But there would be no problem with fitting something like a Manchester M5000 with larger steps to bridge this gap if it didn't have to run on street, allowing use of a nearly standard design. Tram-trains have to have moveable steps for each doorway to bridge the gap when calling at heavy rail platforms.
Tram-train designs seem to be able to fit a 25kV transformer without too much difficulty (or a 15kV one which would be about the same size) so there shouldn't be a problem fitting one in the Metro cross-section. In fact Metrolink probably has a similar overall height limit and definitely has the same platform height, so the same design of tram-train would do for both.
I suspect the old line on the north bank of the Tyne is being kept in reserve in case HS2 ever arrives on Tyneside.
