Is there a shutoff for the dirty hose so...
Is there a shutoff for the dirty hose so that if there is still water in the dirty bag and one wants to fill a bottle from the clean bag, the flow to the filter can be temporarily stopped?
Should a person purchase an extra filter or any other parts as a backup at the time of purchasing the system, or do the parts/filters last through lots of water filtering?
There is one shutoff included with the system. I typically locate my shutoff in the outflow tube between the dirty reservoir and the filter cartridge. I do this, in part, because I use the clean hose and the clean reservoir (with a bite valve) as my hydration bladder during the day.
I do hang the clean reservoir (say, at camp) to dispense filtered water. I simply loop the clean output hose up and over the same branch that supports the clean reservoir (the clean output hose wraps above the water level inside the clean reservoir). The only danger is that the clean output hose would get bumped or blown off the branch and would accidentally drain clean water to the ground (so far, not a problem for me). As you've mentioned, one could move the shutoff from the dirty reservoir output to the clean reservoir output (or simply buy another shutoff) to prevent accidental water loss.
Hope this helps.
Also - If you disconnect the hose from the 'dirty' water bladder, it won't leak/empty out. It's got a one way valve in the connection. I typically take just the dirty bladder to the water source (so i don't drop/contaminate/break the filter and hoses) then lug it up to camp to filter.