Learn English – Is it correct to say “We will provide you with the best consultation and assist you filling out…”

That's a "trick" question because it is not about what follows "best", but simply about "should you use 'in' or 'on'?".

The double-trick is that in some case, you could (with a slight variation of context) use 'for'!

I'll do my best for the TOEFL preparation so I'll get higher score on the exam.

Anyway, if we stick to the initial choice 'in' or 'on', the final trick is in some case, both could be used, like the two first examples ("on" maybe being more commonly used in the US)

I find (personal rule)

• "in" standing for "during" or "within" (or other similar meanings)
• "on" standing for "about" or "referenced by" (or other similar meanings)

Meaning I don't find your examples "wrong".

They are both grammatically correct.

With the same consideration in both cases is an adverbial prepositional phrase modifying the verb treat.

That I would a friend is an adjectival clause modifying consideration. The verb in the clause is implied, and in full form would be that I would treat a friend. While the preposition could be added along with the verb, it is not essential. If it were added, the possibilities are

Am I treating this stranger with the same consideration that I would treat a friend with?

Am I treating this stranger with the same consideration with which I would treat a friend?

While both are grammatically correct (assuming you have surrendered on the avoidance of terminal prepositions), they sound clumsy.

The originals sound fine.