"... What temperature should I combine oils and lye and what should temp should I try to maintain until trace occurs? ..."
Basically you need to keep the temp of your soap batter right at or a bit above the melting point of your blended fats. And you need to maintain this temperature during the time it takes to get the soap to trace. If that is the 115 to 120 degrees F that you mentioned earlier -- and that seems quite reasonable -- then that is your goal.
When you use a stick blender, you can warm the fats until they are just melted, add lye that is pleasantly warm to the touch, and stick blend and hand stir to trace in a short time. Because it generally doesn't take a long time to get to trace with a stick blender, the soap batter is not going to cool a great deal, and you almost never have to add extra heat.
If you take the stick blender away and go to all hand stirring, the time to trace is often much longer. During that time, little or no heat is being created by saponification, so your batter is going to cool from the stirring and from being in room temperature air. This temperature drop can be large enough to slow down saponification enough so if you stop stirring, the mixture will never come to trace or it will only partially saponify. So extra heat may be needed to force the saponification reaction to go faster so you don't have to spend hours upon hours of stirring.
Just a comment -- It is not that important to have the fats and lye near each other in temperature when you first put them together. I know many soap making tutorials and books call for this, but I think this rule-of-thumb may have evolved to new soapers out of trouble (false trace, too-fast saponification, etc.). But once a person has made soap for awhile and has a clue about making soap, this rule-of-thumb is not particularly useful. Kind of like the rule of thumb to only use a sassafras stick to stir soap, or to only stir clockwise, or only make soap in a certain phase of the moon, etc.