As long as tachometer doesn't go to red zone, it's ok to abuse engine break?

As a powertrain engineer with 14 years experience, I can say u have an interresting question. What u are asking is if stopping w the trans and the engine is abusive. The simple answer is yes. When u hard downshift (which is recomemmended for long down hills) to slow down, your transmission will need to synchronize the engine and wheel speeds. It down this through friction plates. These frictions will last the entire life of your vehicle in normal operations. Usually, that is almost entirely accels. By doubling the amount of hard shifts w hard downshifts, u may loose these plates over time. This would lead to a transmission replacement or rebuild. Best to use your cheap to replace brakes to slow the vehicle down. Note that many hybrids use the transmission in this way to an electric motor. These vehicles go through rigorous testing to ensure that the components will live. Other concerns are half shafts which essentually get 2X the damage from these types of maneuvers. GL.