Fly-half Thomas missed a shot at goal, but was on target after another of the Scarlets' Wales squad members, full-back Liam Williams, cut a good angle to power over for their third try.
Thomas added a penalty before Treviso replacement Gonzalo Garcia and Shingler were sin-binned for separate illicit off-the-ball challenges.
De Waal and Thomas exchanged penalties before replacement scrum-half Liam Davies dummied his way through to earn the hosts' bonus-point try four minues from the end.
Liam Williams; Lee Williams, Gareth Maule (capt), Adam Warren, Andy Fenby; Aled Thomas, Gareth Davies; Phil John, Emyr Phillips, Deacon Manu, Dominic Day, Damian Welch, Josh Turnbull, Johnathan Edwards, Kieran Murphy.
L Davies for A Davies (57), R Jones for John (52), P Edwards for Manu (70), Reed for Day (63), Shingler for Welch (48).
Not Used: Hawkins, O Williams, Iongi.
Craig Hawkins, Rhodri Jones, Peter Edwards, Lou Reed, Aaron Shingler, Liam Davies, O Williams, V Iongi.
Ludovico Nitoglia; Benjamin De Jager, Tommaso Iannone, Andrea Pratichetti, Brendan Williams; Willem De Waal, Simon Picone; Ignacio Fernandez Rouyet, Franco Sbaraglini (capt), Pedro Di Santo, Enrico Pavanello, Valerio Bernabò, Manoa Vosawai, Gonzalo Padrò, Marco Filippucci.
E Galon for De Jager (73), A Di Bernardo for Williams (76), G Garcia for Picone (64), C Fazzari for Rouyet (64), E Ceccato for Sbaraglini (64), B Vermaak for Bernabo (64), D Vidal for Vosawai (72).
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