There is nothing in worm castings in the way of nutrition that can't be found in fertilizer, and not only is there no chance it will improve a container mediums structure, as it's fine texture will be detrimental to any medium that is appropriate for conventional container growing. How detrimental is directly related to how much is used.

About mycorrhizae - A) Pots generally offer inhospitable conditions that aren't conducive to the long-term survival of fungi that are capable of forming symbiotic relationships with plants they share the pot with. Populations of beneficial fungi tend toward boom/bust at best. E.g., when I repot in early spring or occasionally in late fall, or when I check roots in winter by tipping the plant out of the pot to examine roots, the pots are generally well-populated with of hyphae of a variety of fungi. Once summer temps and sunshine (sunlight striking the pot and creating heat) come into play, the fungi seem to vanish until temps cool again. I can't imagine that indoor conditions would be any better. Almost all the temperate plants I grow are passively inoculated by beneficial fungi spores which arrive on the wind, and the fungi proliferate under influence of cool/cold temps, but I've never seen the fungi or evidence thereof in a planting that has been growing indoors.

B) All else equal, plants in pots that DO form relationships with beneficial fungi do not need a root system as large as plants that do not form the relationships. Plants are very economical, so if they don't need a larger root system they will not grow it. This means that if something occurs suddenly that reduces the population of fungal pals (like application of a fungicide or other product not appreciated by the fungal friendlies, the root system very suddenly becomes much too small to support the top of the plant, which can cause serious consequences. This is the primary reason I choose not to court growth of friendly fungi in pots. At least for the way I grow, their cultural wants are well beyond what I'm willing to do to try to keep them thriving. I'm growing plants, not fungi.

Be very careful about holding great expectations for products claiming to contain live mycorrhizae. It takes very little heat to kill packaged/living fungi. Some products will contain some live fungi and some none. Some will contain fungi that are unable to live in or adapt to the cultural conditions under which we expect them to prove beneficial. As a variation on the theme "if you build it, they will come", if we establish a container planting and the conditions in the container are conducive to establishment of beneficial fungi, they will arrive and establish quickly w/o our help.

Al