Laptops in this configuration are very rare. Without a discrete GPU the battary life would still be great but you also have way more cpu performance for heavier tasks. Even without the use of heavy applications the laptop would feel more responsive and faster. With a good cooling design it should still be possible to keep the laptop pretty thin, many gaming laptops are good examples for thin cooling designs. This is probably not a good idea for a small 13.5" laptops but in future 15"+ laptops there should be enough space for the additional cooling requirement
This definitely would work, you’d be stuck with poor battery life though as with all the other components installed, there’s not going to be room for a bigger battery.
There is definetly still a lot of space for additional cooling without sacrificing too much space for the battery. A wider cooling thin stack with maybe a third heatpipe that goes to the should be an option. Pair that with a better thermal paste and the laptop should be able to handle a 45W TDP cpu. The ssd could also be moved to the side with the additional space.
Power consumption of the 45W cpus should also be very similar to the 25W options under light workloads and in idle