on you single Undying creature Idea how I understand 601.2c which is relevant due to 602.2b (on "Casting" activated abilities) at the time you determine the number of targets and what happens to said targets you technically haven't even sacrificed your undying creature so targeting it then sacing it as part of the declared modal costs (601.2b) as costs are pay at the time of 601.2h (601.2 is effectively the order of casting spells or going through an activated or triggured abiliity). and Baby Jace techically doesn't avoid this rulling it has the same thing Declare you are ticking up Jace, determine Mode and exact effects of ability if declared ability can be modified so, Select Targets, Decide of the effect is distributed among targets, Pre Resolution Legallity Check, determine cost (adding a loyalty counter), mana abilities ei
Llanowar Elves are activated, Pay cost (tick up Jace),
Harsh Mentor like effects will trigger at this time as the fully "casted" ability is now put onto the stack, Resolution Legality Check, Resolves.