The “r” in “merveilleux” is no different than the one in “merci” or “partir”, it sounds short and is not as rolled as the spanish r.
However, some may intentionally say the first part of words in a long and toned voice to express joy (used in theaters mostly)
e.g: “Mais c’est meeerrveilleux”.
Overall, no the r in merveilleux, if said normally, is not any different.
I listened to the Memrise recording from the link, and I don’t think the r is rolled like it is in Spanish in the recording – it is just the throat sound made when you make the r sound in French. The r sound in merveilleux is the same as any other r sound.