Train/bus from Conegliano-Trieste-Ljubljana

I think you're fine. That will be a regional train, and I think those schedules don't get loaded terribly far I advance. You can't make seat reservations on regionale, so there's no reason to buy the tickets ahead of time. Although I believe there's a schedule change in early June, for general planning you should do fine looking at the current schedule for the day of the week on which you anticipate traveling (assuming it's not an Italian holiday, that is).

For tomorrow, I see trains every hour or two, with a travel time of 2 to 3 hours.

You might also consider scheduled bus service from Trieste to Ljubljana. The Deutsche Bahn lists a 12:55 departure that takes only 1 hr. 40 min. There are additional options as listed on the website maintained by the Trieste bus station. I've taken a number of Croatian buses, and they were all quite comfortable. As a general rule, it's prudent to buy border-crossing bus tickets ahead of time. The bus trip listed by the DB requires a seat reservation. Also keep in mind that it can be difficult to be absolutely certain of bus schedules until you arrive at the departure city and inquire at the bus station.

Thanks so much for your reply! I'm much more comfortable going ahead with my plans now. I think I just needed a little reassurance in a moment of travel anxiety. And, thank you for the link to bus schedules to Ljubljana. That looks pretty clear cut and, you're right, it should be a better option than the shuttle. When you refer to booking ahead for border-crossing bus tickets, how far in advance should I do this? Once I get to Trieste (will only be there three nights) or before?

I have never bought a bus ticket farther ahead than perhaps 36 hours, but I don't have experience with that particular route and cannot give you a 100% guarantee. I see that you're traveling on a Monday, which I suppose might be somewhat busier than Tues-Wed-Thur. It's been my impression that a fair number of my fellow bus passengers are students or workers commuting for the week. Do Google to be sure you're not dealing with the tail end of a holiday weekend in either Italy or Slovenia. I have no idea when the college scholastic year begins in the two countries.

I'm sure it will work out since there are multiple buses each day.

No holidays that I can find, so I think we're good. We'll be traveling from Trieste to Ljubljana on a Sunday, but the link you included shows buses on Sunday. Again, many thanks to you!

Oops. Sorry I messed up the day of the week. I thought I was looking at the September calendar, but I wasn't!

Two years ago, my friend and I used GoOpti to travel from Venice (driver was waiting at the airport) to Rijeka, Croatia and from Ljubljana to Bologna. Excellent service. Would definitely use again. Our first driver did not speak a word the entire trip from Venice to Rijeka, nor did he stop along the way. We arrived 2 hours earlier than our apartment owners were expecting us!

Thanks for your input, John. That gives me two good options.