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.
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 until a certain point rather than a binary Yes or No.