Re point two: your body will become more efficient at doing a certain exercise as you get fitter, which means you will burn fewer calories - how many fewer isn't necessarily a hugely significant amount, I don't think.

As you become fitter your body is more efficient in general... I am not sure what benefits there would be switching your cardio up in that regard. I'm pretty sure that over time your body would just become more efficient at doing different exercises and it wouldn't help keep up calories burned in any significant way.

Your weight also affects how many calories you burn, so as you're losing weight you'll also burn fewer calories.

You should be pushing yourself every workout - if it becomes easy then you're not working hard enough. High intensity interval training is a good way to keep up the intensity of your workouts.

If you're counting calories then it's all about watching what you eat - it is difficult to do enough exercise to make up a deficit, and many machines way overestimate the number of calories burned in a given exercise.