>1 [original] : Je te demande donc de nous servir de guide, mais **uniquement** de guide, et **seulement** jusqu’au lac.

« Uniquement » and « seulement » might seem like synonyms on the surface, but I can’t help feeling that swapping their positions around in this particular sentence sounds odd, if not incorrect.

>2 : Je te demande donc de nous servir de guide, mais **seulement** de guide, et **uniquement** jusqu’au lac.

Now, what about « exclusivement »? Can you use it in both places, that is, leaving aside the question of repetition?

>3 : Je te demande donc de nous servir de guide, mais **exclusivement** de guide, et **exclusivement** jusqu’au lac.

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To my mind:

***« uniquement » :***

Used in a distinctly yes-or-no situation. In the 1st original sentence, « uniquement » is coupled with « de guide » in order to express the idea of:

Uniquement / only as our guide, but not as anything else. Yes or No.

***« seulement » :***

Used when a statement is true only  to a certain extent / partly. In the 1st sentence, « seulement » is coupled with « jusqu’au lac » in order to express the idea of:

We accept you as our guide seulement / only until we get to the lake, but not any further. Partial acceptance rather than Yes or No.