If I understand it correctly, sphere selection is an earlier step in the world creation process from which a number of world effects are generated, including magic (although not the only source, as magic prevalence and potency are slider settings).
I'd expect there to be both single sphere magic (effects) as well as multiple sphere ones, with various pre conditions, where some spells/rituals may only be possible in locations where all the required spheres have a sufficient influence, for instance, while others can be invoked regardless of active sphere influences (possibly barring opposing sphere influences), using thoughts, focii, etc.
You can't assign all objects to spheres in a reasonable manner. One reason is the fully mundane world, where there are no spheres at all, and the other issue is that the spheres present in a world depends on world gen, so many, quite possible most, spheres will be absent in many (or most, or all) worlds (with the selection of the spheres present being different in different world, of course).
However, it's quite possible items might be tagged with sphere associations that might have some chance of being realized if the specifice sphere is present. E.g. an anvil may be an ordinary anvil, but in some worlds it may also have some special relation to fire, while in another (or the same one) it may have a relation to metal and/or artifice.
In most magic systems I would expect associated object (or shape or color, for that matter) to just have a potential, i.e. dropping an anvil on something would cause physical damage (that may or may not be resisted), but to invoke a sphere relation in addition to that you'd also have to do something to activate the relation.
I don't see why Myth & Magic would be tied to introduction of fortress light sources at all. Adding light sources to Fortress Mode would result in adding a need to set them up (player chore), and possibly maintain them (workforce drain), as well as additional burden on the display system (shading tiles depending on the amount of light on each tile), causing display FPS drain.
A further question is what purpose it would serve apart from the aesthetic? I certainly can see a use for it in Adventure Mode, but there's limited use for it in a Fortress. The accelerated time means sneaking under the cloak of darkness for nefarious critters doesn't work well on the surface, while the general stealth system currently used by weres, kobolds, etc. does work without regards to the time of day (which isn't readily available to the player anyway).