Regarding Par station's new footbridge, there could be extra logistical issues erecting the replacement structure - and maybe the temporary one too - on site. If you look at where the existing one is situated, where would one suppose the existing structure be sited? The ideal position has to be where the existing one is, so any temporary structure will need to be erected and gradually replaced or positioned elsewhere. There isn't a lot of suitable real estate at Par for two footbridges. There is ample working space in the old yard but the forcourt is very cramped and the road vehicle access isn't too great either.
On the matter of 4 car trains on the Falmouth branch and being part of the metro system/scheme, platform 3 at Truro would probably facilitate 4-car trains to/from Falmouth, being split there - reversible working back to Falmouth has been provided for out of P3. Additionally Falmouth Town I believe has issues being restricted on limited level ground - may be something to do with why Fal. Docks was reinstated back in the 1970s if memory serves.
Perhaps some of the new rich landlords in the Penryn area could contrtibute towards a proper 2-platform station at Penryn? With a footbridge? Having helpful staff with boarded crossings at stations has been out of favour for almost 2 decades now, as it reduced the operating liability - and besides, reversing that policy would probably be attacking one ideology too far for the UK Government - that of less staff and less skills and more automation.