You probably know this already, but it’s absolutely impossible to answer your first question. Nobody knows what’s going to happen. The border regime could get relaxed at any time, it could be two weeks, two months or even more. It may apply only to local residents living close to the border, it may apply only to Italian citizens, it may apply to all Italian residents. For example if you’re American you might not be allowed to cross even if you might be living in Italy. Then there’s the quarantine requirements etc. etc. You should hope for the best and prepare for the worst, honestly. Right now, the epidemic in Slovenia is over, we’ve had zero deaths and zero new cases for almost a week and before that it was a handful of cases since about late April. The government isn’t too keen on opening up towards Italy too soon and the decision will depend on the state of the epidemic in Italy.
About your planned visit, Ljubljana is a good base to explore Slovenia. The city itself can be seen at a leisurely pace in about two days. A classic tour of the city would include a stroll along the riverbanks, exploring the Old Town, visiting the cathedral, hiking or taking the funicular up to the castle, visiting Tivoli Park. The Metelkova alternative culture centre is worth a visit for the nightlife on weekends (depending on the event restrictions in place during your visit). Then there is the fascinating Joze Plecnik House, if you’re interested in the city’s history or want to learn more about how the 20th century unfolded in Slovenia there’s the City Museum and the Museum of Contemporary History. Walking, cycling and food tours of the city are popular.
Outside of the city, there’s lakes Bled and Bohinj and the Triglav National Park, the Soca Valley, the caves, Piran on the seaside, Logar Valley, many wine regions to explore etc. Some of these places can be visited by bus or train but renting a car would be best, especially given the restricted timetables of most public transportation due to the epidemic. Two weeks gives you plenty of time to explore the country at a relaxed pace.