Everything is checked by the silent check process.Yes, right now players can "appeal" to the VAR (unlimited times) by crowding the referee. In the same Liverpool vs Man Utd match, Maguire was pulling Van Dijk's shirt in his own penalty area. Van Dijk only appealed for a corner kick. I feel that if the Liverpool players crowded the referee appealing for a penalty kick, a VAR check would've been launched. And, I believe the result would've been a penalty (the shirt pull was blatant).
It's possible that a more vigorous approach might have initiated a review but if its as blatant as you say then that should have happened anyway