Soft Filleting and Filling Wax

Yes, it's fine. No cure inhibition problems.

Yes it is a beeswax based product. 

Yes, P2 Fast Cast is not known to react with the filleting wax nor does it stick to it, so you can use the two materials together.

Definitely not! The filleting wax is not suitable for high temperature use and should not be used in this way. The main problem being that the wax would melt at elevated temperature in the oven, especially under vacuum pressure.

Our Soft Filleting wax has a melting point of 60°C.

For Plasticine you state that: "Although...quite firm at room temperature, it is still not rigid enough to use as a mould material in most situations. Any gelcoat applied to it would be liable to cracking if the mould flexed when reinforcement was added"

The situation would be quite similar for wax so, much like plasticine, we wouldn't really suggest using either of these materials to 'sculpt' a pattern which you will then lay mould making materials (gelcoat and reinforcement); however, it would depend on the size and shape of what you're making and how careful you were with the layup process. I could imagine being careful enough with the gelcoating and laminating to use either of these materials as a sculpted pattern if one was very careful about it.

Yes, in most cases the wax can be recovered from the mould/pattern and reused. Over time it will pick up dirt and as such will have a reducing lifespan as it gets dirtier, but generally speaking, clean wax can be reused a number of times.