Sanskrit and Vedic

I have been making a start at learning Sanskrit (including struggling
with the mysteries of the Devanagari alphabet). I have textbooks of
Classical Sanskrit and also of Vedic Sanskrit. How are educated
Indians likely to react if someone writes in Sanskrit and lets Vedic
grammatical features and word usages get into his writings? (For
example, writing [Rta] instead of [dharma], or using Vedic-type verb
infinitive suffixes instead of [-tum]?

What is the situation with Sanskrit prepositions? My Vedic textbook
has a good list of them, but my Classical Sanskrit textbook does not.