Wa alaikum assalaam wa rahmatullahi wa barakatuh,
1. An example of rules changing due to the order of the letters would be a saakinah. If it is followed by one of the six throat letters, it is read clearly. If however, it is followed by a different letter, the rule changes to an idhgaam, iqlab, or ikhfa’,. Another example would be a medd letter. If a medd letter is followed by a hamzah we have either or and we will lengthen it four or five vowel counts, if on the hand it is followed by a shaddah, we will have . The same holds for many rules of tajweed.
2. It seems you are confusing the sound on the shaddah with the sound of the qalqalah. A letter with a shaddah is pronounced as two letters, each pronounced with a different mechanism, the first with a collision of the two participating parts of articulation, the second with a separation of the two participating parts of articulation, assuming we are continuing on a non-qalqalah letter and also when stopping on a qalqalah letter.
In the case of the qalqalah letter with a shaddah and we are stopping on the word, again we have collision of the two participating parts of articulation for the first letter, and separation of the two participating parts of articulation for the second letter without any accompanying jaw or mouth movement. See: http://www.abouttajweed.com/the_mechanism_for_formation_of_letters.htm and http://www.abouttajweed.com/qalqalah_mechanism.htm
The qalqalah kubraa itself is not stronger when there is a shaddah on it, but because we have two letters (referring to any of the five qalqalah letters) that have strong characteristics, one being formed by collision and the second formed by separation, the strength of the two letters (of the shaddah) will be apparent in the sound of the collision followed by the separation.
You need to separate the sound of the qalqalah itself (formed by the separation of the letters) from the sound of the collision and then separation that occurs because of the shaddah. The qalqalah is formed the same way and with the same strength when it is the last letter and we are stopping on it, whether the letter has a shaddah on it or not. The letter with a shaddah itself though, is formed twice, once with collision, once with separation, thus giving strength in the sound of the letter because of it being heard twice.
Insha’ Allah this makes this clearer to you.
Wa iyyaaakum. Wa assalaam alaikum.