Regarding the question of "since" versus "because":
 
Their grammatical function is identical:  they are both clause
conjunctions.
 
Every time there is more than one clause in a sentence, there is
always another clause conjunction to integrate it in.
 
But do "since" and "because" mean the same thing, or is "since"
limited to time?
 
The FUNCTION of "since" and "because" is identical, but their MEANING
might be different; that is the issue in question.
 
Does "because" join two clauses with the meaning of causation, while
"since" joins them with the meaning of time?
 
Does "since" have to mean time, or does it have more than one
meaning, like most words in the dictionary?
 
The question of how "since" and "because" FUNCTION is a grammatical
question; but the question of what they MEAN is not.
 
Convention may change the meaning of "since" to overlap with
"because."  Stylistically, one or the other usage may be preferred.
But the question of function is not open to debate or change.
 
Every time there is another clause in a sentence, there will be
another conjunction to integrate it in.  This is a fact of structure
and the construction of conceptual meaning.  The stylistic or usage
evolution of the word "since" is much more open to elaboration.
 
For what you should tell your students:  tell them that for every
additional clause in a sentence there will always be another clause
conjunction to integrate it in.  Ask them to do a structural analysis
of sentences (eg., circle all clause conjunctions).  Increase their
structural awareness as an end in itself.  Non - grammatical issues
do not even have to enter the discussion.
 
Yours, Robert Einarsson