Learn Dutch: Wat een schijnheilige kerel! - What a hypocritical bloke!

It is a compound of schijn (appearance, illusion) and heilig (holy). So a schijnheilige person is someone who puts on an appearance of holiness — a hypocrite!

Why does schijnheilige get an -e at the end even though it follows een?

Because kerel is a de word (de kerel). In Dutch, an adjective before a noun always gets an -e ending unless it is a het word preceded by een (or no article). Since kerel is a de noun, its adjective must take the -e.

Is the structure Wat een... used exactly like "What a..." in English?

Yes, for singular nouns it works exactly the same. However, a tricky difference to remember for exclamations is that Dutch also uses Wat een... with plurals (like Wat een problemen!), whereas English drops the "a" and just says "What problems!".

How informal is the word kerel?

Kerel is widely used in casual, everyday speech and translates best to "guy" or "bloke". It is perfectly fine for normal conversation among peers, but you would use man in more formal or polite contexts.