Whether a PAP (personal auto) or BAP (commercial auto), the same rules & laws applies when resolving auto claims. The only difference is the amount of liability carried by each...BAP's are generally much higher than PAP. So to answer your question, NO a commercial carrier will not "replace" your totaled out vehicle. Both carriers have a duty to settle claims fairly & do their best to keep their insured out of court.
As for the settlement of a total loss....the law states if it's cheaper to total the car than repair it because the repairs exceed the value of the car, then the car must be totaled. Financially it doesn't make sense, whether it's your neighbor paying or some company with deeper pockets than your neighbor. You do have the option to "retain salvage," where you keep the car & you receive the value of your vehicle less the salvage amount. At that point you can pay to have it repaired if you want but the insurance won't give you additional money during the repair process to pay it b/c you have been compensated...you do not get to benefit from both.
My professional experience with commercial carriers has been they penny pinch right down to the last penny, and rarely ever do they directly pay a claim without it going thru subrogation first. On the rare chance they have stepped up and taken the repairs on first hand they have been slow to respond, slow to issue payments.
If your car is a total loss, have BOTH carriers provide you with a t/l offer and go with who ever gives you the higher amount.