Regional train tickets

On trenitalia.com, you book a regionale train from Milano Centrale to Varenna departing on that day at a specific time.
That ticket can be used on the booked regionale and on any other making the same route and departing within 4 hours. This way you have nothing to worry about, even if 20 minutes weren't enough to make a short level transfer at Milan's station you can always jump on the next regionale to Varenna.
There is no "7 days" rule.
Regionale tickets purchased online do not need to be time stamped, neverthless I'd buy the Milano-Verenna leg ticket at Monterosso time-stamping at Milano Centrale before getting on the Regionale
The ticket purchased online and those at the station have exactly the same life time: 4 hours.

There is absolutely no advantage to purchasing a Regionale train ticket on-line so buy it when you need it. When you buy on line you must specify a starting time and then the ticket is only valid for a four hour window when it must be used. Outside the three hour window the ticket is no good. I think the 7 day rule for local purchase has been shorten to something closer to two or three days. The paper ticket purchased at a machine or ticket window has a shorter life but much longer than an on-line ticket. With a paper ticket, you validated (time stamp) it in a little box near the platform and then that ticket is valid for five hours. Watch the locals do it. Don't buy Regionale only tickets on-line.

We will be in Milano Centrale with an hour wait to board the train to
Monterosso. When we return to Milano to board a regional to Varenna,
we only have a 20 minute span to get to that train. I want to get
tickets for the Milano-Varenna leg ahead of time,

Frank covered the details but you can purchase your paper tickets tickets in advance so that they're ready to validate and go when you board the train to Varenna (I think it's the 20-minute turnaround that worries you?) Just buy them in Monterosso the morning you depart for Milan.

You could also buy the tickets to Varenna at the Milan station on your way to Monterosso, when you have an hour to wait around.

I believe I've read on this forum that there is nowhere to buy tickets in Varenna, so the return ticket from that town needs to be purchased elsewhere. Am I mis-remembering?

No, no. A ticket purchased on-line and printed out comes validated. You cannot validate a ticket printed on a sheet of paper because it will not fit in the validation machine. Only Regional tickets purchased at a ticket machine, ticket window or local travel agencies can be validated. Actually a piece of stiff, almost cardboard like, that is small enough to fit into the validating machine.

Think old IBM punch cards.

You don't need to print out regional trains tickets purchased on trenitalia.com, you can show the QR code using a display.
There is a tobacconists that sells regionale tickets near the main street of Varenna, plus the travel agency inside the station.

"I believe I've read on this forum that there is nowhere to buy tickets in Varenna, so the return to ticket from that town needs to be purchased elsewhere. Am I mis-remembering?"

You are correct.

No, sorry, she is wrong
Travel agency:
http://www.trenitalia.com/tcom/Treni-Regionali/Lombardia/Agenzie-viaggio-Lombardia
Varenna's station is listed as Perledo.
Tobacconists:http://locator.sisal.com

I didn't clarify that I want to prepurchase the ticket to Varenna at Milano Centrale during our hour layover for the train to Monterosso. I am not going to buy it online. Since I would be buying it about 4 days before I would use it, I want to be sure it would be valid on the day and time I want to use it. I thought I had read somewhere that a regional ticket purchased at a kiosk or at a ticket office I good for 7 days. Can I purchase a regional ticket for the Milano-Varenna trip in Monterosso? If that is true, that probably would be the best option.

I didn't clarify that I want to prepurchase the ticket to Varenna at Milano Centrale during our hour layover for the train to Monterosso. I am not going to buy it online. Since I would be buying it about 4 days before I would use it, I want to be sure it would be valid on the day and time I want to use it. I thought I had read somewhere that a regional ticket purchased at a kiosk or at a ticket office I good for 7 days. Can I purchase a regional ticket for the Milano-Varenna trip in Monterosso? If that is true, that probably would be the best option.

Barb, if you’re taking a smart phone, you can download the TrenItalia app. Buy when you need them (soon before boarding) using the app. You’ll receive an email with a scannable code. Just show the conductor your phone and she’ll scan the screen.

Barb,

You should certainly be able to buy your tickets from Milano Centrale to Varenna whilst in Monterosso, although the rail line that goes past Varenna is operated by Tre Nord. There's a full service ticket office in the Monterosso station, and I've bought many tickets there. It's no problem buying them a few days prior to travel, but be sure to specify the date as the tickets will be specific for only the day listed on the ticket. Of course, don't forget to validate prior to boarding the train on the day of travel.

If you need to buy tickets whilst in Varenna, this is the travel agency that is referred to above - http://www.tivanotours.com/

The trip from Milano Centrale to Varenna is ~1H:03M. Sit on the left of the train (when facing forward) for the best views of the lake as you approach. The train doesn't stop for long in Varenna, so be prepared to disembark quickly.

I thank you all for your advice. I think I will investigate buying the tickets in Monterosso. I will know the date we will need, and if I can talk to someone in the ticketing office, I will be assured I am purchasing the right ticket. Thank you for the info about where to sir on the train! I am so psyched to see Lake Como!