Why is Madd Silah called as such, i.e. what is the word ‘silah’ referring to exactly?

Question
As-Salaam Alaykum

Hope you are well in Eemaan and health.

Q: Why is Madd Silah called as such, i.e. what is the word ‘silah’ referring to exactly?

Q: Does madd Laazim only occur when theres a Madd letter followed by Sukoon/Shaddah in the SAME WORD. If so, how comes words like Aalaana and Aallaah are 6 counts? I am referring to the Hamzah of Istifhaam which is next to a Hamzat ul Wasl (which is dropped due to their meeting).

I hope I was clear with my questions.
JZK

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh.  Al-hamdu lillah we are well and in the blessings of Allah and pray you are also. 
1. is named such as it only is heard when ( direct object or possessive pronoun ) connects with the next word in  a lengthened vowel when continuing (wasl) .  It does not have a lengthened vowel when stopping.  So it is referring to the continuation (Wasl).

2. For the reading of Hafs, yes, a medd laazim only happens within one written word, which includes the separate letters at the beginning of some surahs written joined as one word-such as .  The questioning hamzah enters the noun and the hamzah wasl is lengthened (not dropped) six vowel counts to separate the question from the noun.  If the hamzah wasl was not changed (there are two allowed ways of reading this) one would not be able to detect if this was a question or a normal noun starting with alif laam at-ta'reef. 
For more details including the second allowed way of reading the words such as , please see (scroll down please):

http://www.abouttajweed.com/hamzah-al-wasl/hamzah-al-wasl-lesson-8.html