As I've said before (often), 'la' does (among other things) allow moving prepositions from the end of the sentence to the beginning, usually with the same meaning (but not always, see 'tan ni', where the 'ni' picks up different referents by its position). And, yes, 'lon' is usually lost, especially with 'tenpo'.
The purported loss of 'lon ni' is not convincing, since there seems to be no reason to think it was ever there to begin with (although it does provide some connection between the two main uses of 'la', which is nice.)
The interpretation of 'pona la' is ruined by 1. nobody says the purported source and 2. the expression doesn't mean that.
I note in passing that none of the things here noted as odd are in pu or follow in an obvious way from what is in pu.