Is there a way to lower the speed at which the EV is disabled? I would like less EV mode when driving to preserve the hybrid battery. Right now my 2012 Prius V may turn off the ICE below 43 mph. I would like to lower that to say 25 mph.
It's all about gaining read & write access to the firmware code... Toyota has kept it locked down better than most. If we were talking about Honda, there would be lots of interesting ways to access and modify code but Toyota is entirely awful when it comes to collaboration because they think it would interfere with their profits on many different levels. Basically when it comes to Toyota your best option is spoofing, which means if you can make the computer think you're going 43mph when you're actually going 25mph you can get the result you want. But as you might imagine, that probably is going to cause other problems elsewhere in the system and you'll have to spoof to resolve those problems too. Basically just about any change you try to make is gonna create warning lights and related system shut down. As far as I know the only people who have been successful is TRD, which once converted a Gen2 to run on a bigger electrical motor, but because they are an extension of Toyota, they had access to rewriting the code.
Even in POWER mode it will REGEN in electric mode below ~43mph, but that's not the OP's issue. When accelerating in EV mode below the ~43mph cut line, watch the dash display to stay just below the torque load that calls for the ICE to fire up.
It won't prolong the life of a hybrid battery. Even if it's not helping the ICE to propel the car, high voltage is used to run the ac compressor and it's always generating 12v power for vehicle low voltage loads. High voltage is also used to operate the transaxle almost all the time. Plus it is required to accept power generated by the ICE to recharge itself and it must take large brake regen currents.