It is perhaps not easy to work out the relevant meaning differences if you only compare advenire and venire. In order to better understand the role of the prefix, it can be useful for you to consider the "full" picture/system, which involves comparing advenire with other prefixed verbs like pervenire, obvenire, evenire, etc. You'll then discover that each prefix adds a particular directional/path nuance.
As for the repetition of ad as a preverb and as a preposition in your example, it should be noted that this phenomenon is typical of so-called "weak satellite-framed languages". Latin and Slavic languages are classified into this typological (sub)set. Please take a look at this link for an answer to a related question on motion verbs.