Be careful with the term 'usable fuel'. In essence that means everything in the tanks. So not necessarily but if you're planning to land at MLW (perf. or structural) then you would need to burn the trip + cont. You can't plan to use contingency fuel and you can't always guarantee it'll be used so if you're close to the limits then you have to be careful with what you take.
Alternate fuel is accounted for in the sums - MLW + trip = MTOW. There is no need to burn the alternate fuel to ensure you meet MLW at destination. The plan is to always arrive at DA/MDA with at least ALT + FRF but if you calculate that you will arrive with less then you are allowed to continue to destination assuming the weather and traffic situation is agreeable.
FRF is there to be used if you need it, in euroland it's the final 30 minutes worth. If at any point you calculate you will land with less than FRF then you declare a mayday.