[Big-Daddy]

Thanks. To clarify, is this correct:

My initial feeling is that if we add more reactant and/or product (this can be raising concentration or raising partial pressure, doesn't matter; raising pressure raises partial pressure for obvious reasons) then we will finally have more reactant and product at the end than we would have had if we hadn't added anything;

That itself seems inconsistent. When increasing pressure you are not adding anything new at all. In this case raising pressure will not finally lead to "more reactant" at all. (Assuming a decreasing moles reaction)

I was writing the general argument with relation to concentration. When dealing with gas, we would say partial pressures instead? (Which would be increased if you increased the total pressure or if you added more moles).

I would love to do this using equations instead of words. Let me try and clarify what I wrote: yeah your definition of yield, the fraction of reactants converted to products, seems good.

If we add either reactants or products, then our equilibrium concentrations of both reactants and products will be higher than before but the equilibrium ratio of concentrations of products over reactants will be maintained. Correct or not?

If we increase the pressure (while calculating K_p), or add gaseous moles of either reactant or product, our equilibrium partial pressures of both reactants and products will be higher than before but the equilibrium ratio of partial pressures of products over reactants will be maintained (as per K_p being constant). Correct or not?

Henceforth I will assume arguments applying to concentration and Kc also apply to partial pressure and Kp.

If we add reactant, the reaction will be driven towards the products (as Q<Kc) so equilibrium yield will increase. If we add product, the reaction will be driven towards the reactants (as Q>Kc) so equilibrium yield will decrease. Correct or not?

[Corribus]

If we add either reactants or products, then our equilibrium concentrations of both reactants and products will be higher than before but the equilibrium ratio of concentrations of products over reactants will be maintained. Correct or not?

Correct, to a good approximation.  For the next question as well, if I'm reading it correctly.

[Big-Daddy]

If we add either reactants or products, then our equilibrium concentrations of both reactants and products will be higher than before but the equilibrium ratio of concentrations of products over reactants will be maintained. Correct or not?

Correct, to a good approximation.  For the next question as well, if I'm reading it correctly.

So all of those ones are right?

The problem with yield is that because the calculations can be decently long it's difficult to use the quantitative aspect to visualize the system qualitatively.