- This is your job.
- Defined on the datasheet: "Itrip = Trip current: minimum current at which the device will trip in 20°C still air. In other words it is guaranteed to (eventually) open at that current under the specified conditions. It may open at less current, depending on unit-to-unit variations and depending on ambient temperature. Refer to the derating curve. For typical time to open at fixed ambient refer to the preceding graph (this is important for figuring out how it behaves with short overloads)
- There is really no basis to extrapolate the data, especially since it's not even in a specified direction. Typically that kind of drift is not linear but might be similar (cumulative) for 10,000 hours as for 1,000 hours, but as I said, there's really no information. What you can glean is that this is not a precision device and you need to allow a lot of safety margin.
In general you should try to make the difference between "must carry" and "must trip" as great as possible. That will make it easier to specify a fuse that will not open under extreme conditions (maximum ambient temperature, worst-case current and surge) and yet will protect what it is intended to protect under the opposite worst-case conditions.
The purpose of such a device is typically to protect the copper traces and maybe some other things such as wiring, so making the traces and wires fatter than necessary to carry the normal current will increase the available margin.
Note that the 750mA device will typically carry perhaps only 500mA at 70°C ambient, for example. If you expect the device to operate more than once, there might also be other aging factors involved, suggesting even more margin.
Also, SMT devices are sensitive to the amount of surrounding copper, far more so than through-hole types.