Answering your cycling questions

Pop out the little rubber cover and screw out the little screw. You should be able to pull out the dummy on the front end a bit and slide it out towards the front. Fit the 'real' sensor in reverse order...

Hi, There is a small rubber gromet in the centre of the plastic face - pull this out and then there is a small allen bolt holding the plastic cover in place - undo this and then the plastic cover will come out easily. Put them to one side and keep them for future use. Fit the new sensor with the new allen bolt and rubber gromet - you may need to use a little oil on the sensor arm to push it through.
Once fitted you will need to make sure the crank magnet is fitted in the right place to ensure the sensor is working I found that I needed to put this closer to the pedal shaft than in the instructions- you can do a search on the web for additonal instructions although they are limited. No magnet is provided for the wheel so you will have to buy one - approx £5.
Once set up it works consistently - shame about the lack of instructions and wheel magnet.

There's a little rubber seal that hides a small screw in the dummy you now have in frame.
Remove the seal, unscrew using an allen key to remove the dummy. Then push in the real duo trap and secure it with the same screw (or with the one that came with the duo trap, they're both the same). Then put the seal back and you're done.

Don't be afraid to pull it's tight but will come out it's the same to install just push and always follow the shape of the chainstay

If it picks up ANT+ on 2.4 GHz, then yes

Yes it Is compatible with Any ANT+ Device

Yes, this is ANT+ compatible.

It will only fit to Trek bikes which have the specific recess in the frame which holds the dummy. Since 2011 I think Trek is doing them on all carbon 4,5 and 6 series madone frames. Before that it was only the 6 series as far as I know.

Yes It is! Compatible with Any Madone with Inbuilt DuoTrap Sensor slot

No, it needs the recess - but other brands of ANT+ type sensors that fit in the more traditional way will fit.

with polar no :-(

Polar CS500 is not a 2.4Ghz Ant+ compatible so you can not use this sensot with it.

I'm afraid not, you can use the polar or the Garmin sensors as they are ANT+

Do you have the prepped space with the dummy duotrap on your frame? If not it doesn't fit.

Trip5 is an analog unit. Duotrap is ANT+ 2.4 Ghz digital standard.
Yes node 2 comes with spoke magnet, but any other will work as well.

No, it will not fit an '08 Trek. The newer Trek Madone 5 and 6 series have these pre-built into the frame chain stay. I would recommend another suppliers speed/cad sensor zip tied to your frame.