1. It is hard to predict how long your mycorrhizae will survive. The manure will be a great source of innoculum to the leaves and pine needles. An important distinction to keep in mind is that mycorrhizae are to aid the plants with uptake of nutrients will plants that are alive. The saprophytic fungi are what will break down your pine needles and leaves (this will make great fertilizer).
2. I'd suggest keeping up what you're doing. Make sure there are some plants sort of nearby to increase percentage of endophytes (fungi that will actually aid in uptake of nutrients to live plants). The most important thing is to have adequate dead/decomposing organic matter which you'll have if you passively keep your compost full.
3. It won't stop producing. You may have different strains of fungi take over that have varying degrees of effectiveness, but it'll always be better than nothing.
