Ask again in 10 years. Can tell you then....

I noticed that the rechargeable batteries are slightly thicker than the normal batteries. However, by "slightly" we are talking about fractions of a millimetre. In the case of a template cast to the hundredth, this can be a problem because the bodies do not fit into the shell. I have been using the batteries in various devices for years: Telephone receiver, clockwork, computer mouse, remote controls of HiFi components etc., with me the deviation has never led to complications. But I can understand why there might be a problem with the accuracy of fit in your area of application. I don't see a solution, except that you switch to normal batteries.

I don't think that Power or Endless make much difference here. There are more important factors. I use batteries with a large capacity whenever possible and operate almost all devices that actually need batteries with them. It should be noted that rechargeable batteries only deliver a voltage of approx. 1.2 volts, whereas batteries deliver 1.5 volts. This means that a rechargeable battery has to supply more current to generate the same power and therefore often lasts much less time than "real" batteries. In addition, wireless phones often have charging stations, but these are not very intelligent - which means that the battery suffers in the long run. But these things are the same for all batteries.

The Endless is completely sufficient for a wireless telephone in the home. The capacity depends on the duration of the call. For normal private use, the 750mAh is sufficient for several days of stand-by operation.

Yes, no problem.

Charger is not included, can recommend the Varta LCD 15 min Ultra Fast Charger.

The batteries usually have a size indication, such as AA or AAA. If you are not sure, you can also measure the battery and indicate the diameter and length. The manual, inside the remote control slot or cover, or on the battery itself should normally indicate the designation.