Using WHAT spare capacity in Manchester???
Well, perhaps the 20 trams-per-hour terminating at Victoria or the 10tph terminating at Piccadilly, or the 10tph terminating at Bury, Alti or wherever you want to join the main rail network. All you are doing then is extending tram routes that have already crossed the city centre. Or you could do Wigan/Leigh - Hadfield/Glossop/Rose Hill using capacity that would exist on the Pic-Vic axis...
We're already realistically looking at 40tph into Deansgate when 2CC comes in, this being the upper limit of any implemented signalling system in the world, the highest implemented in the UK is 33tph (On the Jubilee Line and Victoria Line on London Underground, using DTG-R Signalling)...
Yes, but these are trams running line-of-sight (like as if they were buses) through that area, not tube trains, and you can run a maximum of 86tph using line of sight according to TfGM. Which sounds rather precise, and seems weird that they didn't just say 90 (ie one every 40 seconds) - so perhaps they have done the modelling, rather than pluck a number out of thin air.
Oh, and with all the branches, the current limiting factor of the Jubilee and Victoria lines isn't there - turnarounds at ends of lines take time - you might not get that much more, but you can get some more out of them. Though branches bring in reliability issues, but with effectively 86 'paths', there's enough flexibility for 60tph through Cornbrook and Deansgate.
Certainly the Moscow metro runs a line at 40tph, and the District line used to run at that frequency.