I don't understand pretty much any of that post Bishadi.
As for the OP, lets begin with your questions:
"Do I understand correctly that completely non-evasive and passive acts of ‘watching’ or ‘measuring’ really cause a change in how the particles acted?"
The thing is, there is no such thing as a completely passive measurement. In order to gather information about what your photons (or electrons or whatever) are doing (i.e. about which slit they go through), you have a measuring device which interacts with them somehow. It is this interaction which screws up their nice quantum state and leaves them in a more classical particle state which you can then say went through one slit or another. The major point of the experiment is to emphasise the fact that by performing these measurements you are in fact changing the experiment, and therefore cause new results to be observed.
"Exactly how was the barrier ‘watched’ or ‘measured’?"
There are all sorts of ways to do this. One (rather contrived, but sort of useful to imagine) way is just to replace the usual screen with a sort of telescope apparatus hooked up to a photodetector, so you can count every time a photon hits the detector. Imagine the whole thing is in a super-dark room with no extra light and a source that spits out a photon only once every few seconds. You can point the telescope at one slit or the other, and if you get a blip in the detector you know the photon came through that slit. If you do this for a large number of points (i.e. to regenerate the absent screen) and for a long time (so you see lots of photons and get some good statistics) then you won't see an interference pattern, even though you have done nothing to the experiment anywhere near the slits. You can go many kilometers away if you like and have really awesome equipment (slightly impossible in practice)
A simpler method is to "tag" the photons as they go through the slits, so in principle you could check this tag to see which one they went through. For instance if you place a linear polarising filter over each slit, oriented 90 degrees to each other, then the light coming out of each slit will be oppositely polarised and you can tell which slit a photon went through from it's polarisation. The interference pattern will be wiped out, even if you don't bother to actually measure the polarisation of any of the photons.
This may be less satisifying though because it is more obvious that we did something to the photons. However, my first example was not totally unlike just blocking off the slits one at a time so you know that any photon you see came from the open slit, it was just disguised a bit by making this blockage far away from the slits, so maybe you don't like this either.
In any case you can be as clever as you like but the effect remains. One interesting note is that if you only gain partial information about which slit a photon went through then you only partially wipe out the interference pattern.
Oh, as for the many-worlds thing, that is just a way of trying to understand what is happening. We can get to that if you are ok with the rest of what I said
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