I don't know the best practical workflow. But in my current experiment, my workflow is: First, retarget the motion to an armature that has IK and FK controllers and functions of baking IK/FK. For example, Rigify rig, Auto-Rig Pro rig; Second, use this addon to lock the FK foot and recalculate the location of the Hips (torso/c_root_master.x) bone, in this step, we don't care about any sinking to the floor or not touching the floor or how to lock both feet. The main purpose of this step is to recalculate the location of the hips bone; Third, bake the FK motion to IK motion, and then use the IK controllers and/or other constraints to lock the foot. In a word, use this addon to lock the FK foot and recalculate the location of the hips bone, then bake FK to IK, and then use IK and/or other constraints to lock the feet finally.

You have to ensure that the addon has been installed and enabled. And the addon menu only appears when the Object Interaction Mode is in Pose Mode.

It depends on the Lock Type. Head, Center, Tail, these three Lock Types will only recalculate the location of the master/root bone. If the Lock Type is Entire, it will also recalculate the rotation of the Lock Bone. Note that All of the Lock Types will recalculate the location of the master/root bone. 

Yes! It is a one-time purchase. And all future version updates for the product are provided at no extra cost.

The most difference is: The Inverse Lock Bone mainly recalculate the location of the master/root bone to let the foot locked, and the character's pose animation didn't change in most case; The IK constraints mainly recalculate the rotation of the thigh and the shin to let the foot locked, the character's pose animation changed a lot in most case. They are used in different situations.

You can lock one foot and recalculate the location of the master/root bone with this addon first, then use IK constraint and/or other constraints on the other foot.

Check if your master/root bone setting is correct. In the Rigify rig, use the torso bone as the root bone, which is not a problem in FK mode, but it is a problem in IK mode. Use the root bone (the big circle) as the master/root bone if you need to control the ik bone.