Wa alaikum assalaam wa rahmatullahi wa barakatuh,
1. When the last letter of a word has a shaddah on it, and we are stopping on the word, the only way to form the letter is with collision of the two participating parts of articulation. It is impossible to have collision of the first part of the shaddah, and separate before colliding again for the second part of the shaddah, for if we do this, we will be putting a vowel on the first part of the shaddah, which as we know is saakin. Physically we cannot just by normal letter formation make clear that there are two letters on shaddah when stopping on it (outside of the exceptions we have stating in the lesson on the . The only way to make clear that there are two letters (the two parts of the shaddah) is by making a , or accent. This makes clear there are two letters. There is no other way to do it without adding a vowel or qalqalah to the first letter of the shaddah. We stated in the lesson you are referring to: When stopping on a word like this, we stop with one saakin letter, meaning this letter is formed by collision () of the two articulating bodies. It is then as if one letter has been dropped from the reading. It is for this reason that the learned reciters of the Qur’an warn of the necessity of an accent on this last letter, actually beginning on the letter preceding it, to point out to the listener that this one letter, is actually two. We drop the vowel of the last letter of a word when stopping, but do not eliminate the letter.
This means then that the ensures we pronounce the letter as two, not one.
2. We stated in the lessons on the (further down on the page of the lesson referred to in the question) that there is another exception to stopping on a letter with a shaddah and the . The second exception, meaning there is no when stopping on a letter with a shaddah, is the qalqalah letters. The mechanism of the qalqalah is such that each part of the shaddah (when stopping on the word) is formed by a different mechanism, the first letter with collision of the two participating parts of articulation without any mouth or jaw movement, and the second part of the shaddah is formed by separation of the two parts of articulation without any mouth or jaw movement. This is the reason there is no extra emphasis on a qalqalah letter with a shaddah when stopping on it. There is an exception to this exception though, which is if there is a medd letter preceding the last letter of the word and the last letter has a shaddah and is a qalqalah letter; in this case, we will have a nabr just before the qalqalah letter with a shaddah. This is stated in the lesson as following:
Another exception, Allah knows best, is stopping on a qalqalah letter that has a shaddah. When stopping on a word ending with a qalqalah letter with a shaddah, both letters of the shaddah are pronounced. Examples are in the following words: and
When stopping on either of these words both and both are pronounced. The first in the word is pronounced with a sukoon, and the second is pronounce with a qalqalah. The same can be said about the two in the word . The is then left out in this case, unless the qalqalah letter with a shaddah is preceded by a medd letter, such as in: and for it then falls in to the third circumstance of , which will be discussed subsequently, insha’ Allah.
To see the third circumstance of the nabr lesson, please see: http://www.abouttajweed.com/an-nabr_part_2.htm
It would be a mistake to put a in a place not requiring a , in fact we know Qur’an schools here take off points in tests if a student makes a in an inappropriate place. When stopping at the end of an aayah, we need to make collision of the two participating parts of articulation of the last letter (as long as it is not a medd or qalqalah letter). This is what will make the last letter clear. We should not slow down, raise our voices, or otherwise change our recitation at the end of an aayah, unless of course, it is required to make a on the last word, following the rules of the laid out on this site.
Wa iyyaakum. Wa assalaam alaikum.