check for ECU det sensor code.
also check the engine coolant temp sensor resistance is within spec as per the service manual using a multimeter.
if those check out ok then get your O2 sensors checked out.
I would suggest getting the car hooked up to Nissan Datascan to check actual temp. It could just be that the temp gauge needle is not positioned correctly.
yes but you will need a special device to bung up the intake with a bicycle valve, should be a DIY article here somewhere on how to make a boost leak tester.
best way to eliminate boost leak is using compressed air into the intake. boost leaks are never apparent. there are just too many places where boost can leak from.