In most situations は is pronounced "ha". The exception is when は is used as a particle to indicate the topic of a sentence, in that case it is pronounced "wa".
The reason why こんにちは is pronounced that way is because its abbreviation of the phrase こんにちはごきげんいかがですか。but over time it got shortened to こんにちは but the pronunciation stuck. は is always pronounced ha unless a particle or used in an abbreviation (most of them are to do with time)
「こんにちは」,「こんばんは」, etc., aren't a whole word as if it's the spelling, but actually are phrases. The は part is not a "noun". It's a particle; A piece of grammar that gets attached to the word before it.
As explained above, they're actually condensed forms of a whole phrase on their own. It's cut off after は, but the statement is familiar that it isn't fully stated.
