jan Lope o, toki!
I think we agree in more stuff than I expected.
There isn't so much effort to be put on just putting a preposition on another position, without even changing its meaning. Just put it there. End. "la" is not defined in terms of "lon" when not a conditional. It has an unique structure, too: it links a fragment to a sentence.
I tried not to involve multi-type words in the discussion because they can be any of their types. The order doesn't matter. Just like "tawa" in the infamous example, "sama" can mean "equal" or "as". But in this particular case, probably one meaning makes more sense than the other. After all, "sama" before "la" means "equally", like in "mi pu. sama la sina pu." (I pu, and equally you pu.) and so, how would sina modify "equally"? I think a sentence like "as you la I pu." easier than "equally you-ly la mi pu." Observe that "sama la sina pu." can also be written "sina pu sama." And "sama" can't be a preposition here, neither isolated before la. But if you also don't consider "sama la" to mean "equally", then you're negating "sama" is an adjective, because for you its only correct interpretation is that of "lon": "mi pu lon sama." (frankly, I don't see much difference, but only an unneeded transformation of sama into a noun to say the same thing differently, less clearly).
It was not just Tepan who wrote sama is a preposition there. He's got Sonja's word that this was possible. I can't see how "mi pu sama sina." is clearer than "sama sina la mi pu." Actually, I think the opposite: Couldn't "mi pu sama sina." mean "I pu equally you-ly"? What about "mi pu lon sama sina." (I pu on your equalness)? Aren't those the other possible ways of expressing "sama sina la mi pu."? They're more complex in my view. Maybe simplicity is subjective, just like anything in toki pona.