Partial answer for now. I'll address what seems to be your main focus.
Viewing Targum on Shemot 9:11 which has both words לפני and מפני
לפני is translated by Targum as קדם which could "generally" translate as "in front of" (either positionally, as when people stand in a line), or "before" (which could also be positionally as when one event occurs "before" another), or "before" meaning "in the presence of", which is what I believe is the best translation in Shemot 9:11.
מפני is translated as מן קדם meaning "from the presence of". The term מן could mean either "away" (which is what "from" could mean) or "because of". So, in Shemot 9:11 it means that the magicians couldn't stand in the presence of Moshe because of the boils.
Likewise, in the verse you cited, it means, because of שיבה (I'm uncertain how to best translate this word), you should stand.