Train Sanata Margherita to Riomaggiore

A regional train ticket in the Italian province of Liguria must be used in a certain amount of time, less than the 4 hours for most of the rest of Italy. You really would not be buying a round trip ticket, you would be buying 2 tickets, one from Santa Margherita to Riomaggiore and one from Riomaggiorre to Santa Margherita. You would validate the first ticket before boarding the train in Santa Margherita which starts the clock on that ticket. Then on the return, you would validate the other ticket just before boarding the train in Riomaggiore.

So you can hop off and on for the validity period of the ticket.

I don't think "hop on hop off" is technically allowed even within four hours, but I doubt they would be able to check.

In any case, in a practical sense, I'm not sure you're going to have enough time to hop off and on at Manarola, Coniglia, Vernazza, and Monterosso anyway and still have time to explore the towns in any reasonable amount of time. Corniglia for one is up the hill from the train station - you have to hike up the hill or take a shuttle bus. Also, many of the regional trains heading north from the CT villages are "Cinque Terre Express" trains that don't go all the way back to SML. If you attempt something like this, plan carefully so that the last train you get on does indeed go all the way back to SML or you will need to change somewhere in between.

Since it's likely you'll need a Cinque Terre train pass (or combination hiking/train pass) anyway for your visit, here's another idea to consider. Buy round trip regional train tickets to Levanto instead of Riomaggiore. They're only 4.30 euro each way versus about double that to Riomaggiore. The Cinque Terre train pass is good for all day travel between Levanto, Cinque Terre towns and La Spezia, so buy your train passes in Levanto before continuing on to the Cinque Terre towns. Most if not all regional trains departing Monterosso will stop in Levanto 5 minutes later. I think you can board a regional train heading to Santa Margherita Ligure in Monterosso and use the Cinque Terre train pass for the short segment to Levanto, but you may have to hop off quickly in Levanto to validate your return tickets if not pre-purchased (i.e. already pre-validated).

Google translate from the Trenitalia Conditions of Carriage for Regionali

Intermediate stops - The traveler can make an unlimited number of stops during the journey validity of the ticket.

Like I say, that is 4 hours in most of Italy, but I think in Liguria it can be as short as 75 minutes.

The regionale Liguria time limit is 75 minutes up to 30km and 4 hours after that. Trying to use the 75 min to save a ticket costing around €4 is unwise.

Tickets from SML to Riomaggiore [€13] and returning from Monterosso will cost approximately €21. Stops in between cost €4 each. So e.g. if one stopped in Manarola and Vernazza, total cost for the day would be about €33.

E.g. at 8:26AM it's €9 to Levanto; changing trains to Riomaggiore or any other village €4.

A €16 one-day CT card for trains only would be a poor investment.

Another more expensive but more scenic option is to take the ferry from Riomaggiore back north to Monterosso. The ferry doesn't stop at Corniglia though and doesn't run as often as the trains. But you do get a great view of each town from the water.