Answering your cycling questions

Yes, The sensor for cadence and speed/distance is mounted on the rear chain stay.

The sensor must be mounted on the rear wheel anyway as it doubles as the cadence sensor.

Yes

The V3 sensor it mounted onto the rear stays as it measures the Cadance as well as speed through a joint sensor. It'd struggle to do so on the front fork. so yes it is perfect for use on an turbo trainer!

Depending on the frame, no reason why not - the range of the wireless should be sufficient.

I have a cat 2 series roughly the same as this one I use mine on a turbo trainer the things you have to be careful of are most cycle computers are made for front wheel fit the more modern cordless one have better fire free range than before the one I have doesn't have as many features as this but it works with no problems you may want to purchase the extra fittings from cateye so you can use it on both also go and see one in store if you can and read the section on fitting this should help..

No - everything required is contained in the box. However, it is good to note that you can buy spare components such as wheel magnets

Everything you need is in the box. You don't need to buy anything else.

The Cateye V3 Package comes with everything you need to install and everything listed on the product description. You don’t need to buy anything. Ready to use.

NO, it is all included in the Cateye V3.

Yes, the heart rate sensor work well when you are resting or off the bike. I think you can use the heart sensor to run but you will need to hold the cateye all the time

yes

Yes it can. It's a bit spooky for some reason when you are just walking about with your monitor on getting ready or something and you notice you are being measured by the machine.

Yes