Why did HVAC company add a heat outlet register venting into my attic?
&quot;Too much air&quot; would not trigger overheat switches.  Too much heat would.  One possible cause is an oversized furnace with not enough registers or not enough return air or both.  This could cause overheating as you describe.  It's a guess.
[EDITED to add 3 additional, less drastic, steps] : There are some things you should try, that I hope you or the installers tried before installing the new attic register.  You could try them now by plugging up the new attic register with cardboard and duct tape.

Make sure the filter is clean.  If you have a high MERV filter (8 or higher) you need to change it frequently even if it doesn't look dirty. Try removing the filter entirely just for a day to see if that alone cures the problem.
Make sure all the registers are open.  Does that cure the problem?  If it does you may just have to live with it that way.
If not, leave all bedroom doors open.  Doors that are tight all around, eg that contact carpet at the bottom, prevent air flow from the registers to the return.  If leaving them open cures the problem, you need to shave gaps in the bottoms of the bedroom doors or install grilles to increase airflow.

If none of those things work the solution is to install a smaller furnace, or to add ducting and registers (in and/or out) to improve air flow through the furnace.   My guess is your installer did that ... the easy way.  They added a register, with no new ducting, that just dumps air into the attic to increase flow through the furnace.   Presumably there is no return air path in your attic so in addition to heating the attic and stirring up insulation and mouse droppings they also created air flow through the attic hatch and/or any other possible path from the attic to the return register that is probably in your upstairs hallway.
It's a guess, I hope I'm wrong.   Other than replacing the furnace with a smaller one, the best solution would be to add registers in the rooms being served by this unit, but that may in turn necessitate more or larger ducts both to and from the furnace.
Hopefully my guess is wrong and your problem is something else entirely. :(.