One of the best ways of removing waswasah is never to accommodate to thinking of such scenarios. Do not create scenarios in your mind, and do not assume najasah at all. Your taklif and duty is to completely dismiss your shakk by ignoring it and considering whatever scenario you are in to be tahir and pure.
If you have no concrete evidence that an item has really and definitely become Najis, you can consider it Taahir as well as all items which touched it. Precaution is good good but it is not obligatory.
Not in most cases. Wiping would not suffice when purifying, and "washing" must be done, even if it is a very small amount of water, as long as it is "washed".
There are certain laws related to using something other than water when doing istinja` from extretement of faeces. The details of this you must read in the book of laws of your Marja' taqleed.
Doubting whether the Najasat has transferred to your body or cloth or not, does not make your body or cloth Najis. You can consider everything as Tahir unless and until you know for sure that it is Najis.
'If you are sure that you performed a Prayer while your body or cloth were Najis, then you need to be Tahir and perform that Prayer again.
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