Itinerary advice for 3.5 days in Slovenia

If you haven't vetted your driving times on ViaMichelin.com, I suggest doing that. I did a quick check on Rome2Rio.com, and the trip looks theoretically possible. I'm a bit concerned about your plan to rent a car Monday morning and cover so much ground (with stops) that day, because some folks have said that picking up a rental car in Europe is not the 10-minute process we are used to. But I have no experience with that, and it may indeed be a quick matter to get a car in Ljubljana and get on the road.

But do be careful with the Škocjan Caves leg. In fiddling with the routing I found, to my surprise, that there is a town (I guess) called Škocjan that's SE of Ljubljana, on the way to Maribor. The Škocjan Caves are SW of Ljubljana. Confusing the two would be unfortunate.

I think you'll be OK in Ljubljana on Sunday. The castle and some of the museums are open 7 days a week, and at least some of the 6-day places close on Monday rather than Sunday. If there are things you especially want to see, check their websites. Incidentally, the castle can take up quite a bit of time, and to me it's skippable if you only have one day for Ljubljana. I'd rather spend more time walking around the historic district. But others may well disagree. And though I usually enjoy boat trips, I was disappointed that the one I took (free with city card) did not have the promised multilingual commentary. In fact, I don't remember any commentary at all. You can get a good view of the area around the river by simply walking along both sides of it.

Ljubljana has a flea market on Sunday morning. If you want to see the produce market, you'll need to take a quick look Monday morning before heading to Lake Bled, because it runs only Monday-Saturday.

I'm sorry I can't really address your specific questions, but I took public transportation in Slovenia and don't have a good sense of how much ground can be covered in a day when you have a car.

Let me give you a rundown of what I did in May 2014. (I'm about in your age range.)

I arrived in Ljubljana on Sunday, May 4, after a flight from the USA. I don't remember finding things closed that day.

On Monday, May 5, I explored Ljubljana for the day.

On Tuesday, May 6, I did a day trip to Bohinj and Bled and returned to Ljubljana. I drove on the scenic back road to Bohinj (via Škofja Loka), and that was the most gorgeous scenery of the entire trip.

On Wednesday, May 7, I did the drive to Kobarid over Vrsic pass. (The pass had opened about a week before.) I stopped on the way at the beekeeping museum in Radovljica. I stayed two nights in Kobarid. On Thursday, May 8, I did the Kobarid Historic Trail hike, visited the WWI museum, and had time to relax in Kobarid.

On Friday, May 9, I drove to Tolmin, visited Tolmin Gorge, then drove to Skocjan Cave and toured the cave, then drove to Piran and spent about an hour there, and then drove to Rovinj.

You have about 2 fewer days than I had to do most of what I did. Personally, the first thing I'd skip of all the things I did was Skocjan Cave. Second, I think I'd skip Piran, though I liked it a lot, but it's not that different from Rovinj. Third, I'd skip Bohinj. I would not skip Tolmin Gorge. (I should tell you that when I was there, Vintgar Gorge was closed, so that wasn't an option, but the TI in Kobarid told me that Tolmin was even nicer, and I thought it was great, and it took less than an hour.)

So I guess in your time frame, I'd do three nights in Ljubljana and one night in Kobarid. You probably won't have time for the Historic Trail, but you should see the museum. If you're absolutely committed to Skocjan Cave, go for it. It's very pretty, but it was very touristy and photos are not allowed in the cave, and they move you through pretty quickly. For me, at least, it was not a highlight of my visit to Slovenia.

Where are you planning on dropping off your rental car? Are you heading back to Slovenia? You realize that there is a substantial charge for dropping it in a different country. (I flew into Zagreb and rented the car there and dropped it off elsewhere in Croatia later on my trip.)

I will try to answer question 2. I think that one day without at least staying overnight is not enough for Bled Bohinj and Vintgar Gorge. Especiallly because you have to drive from LJubljana and back, which makes it half a day in fact. I was there 4 days in july and visited exactly the same Bled (where I stayed) Vintgar Gorge and Bohinj. In this film you can see all three:
https://www.youtube.com/watch?v=8pL0CVKTzW0
Theoreticaly you can see all of that in one day but you couldn't feel the atmosphere, that would be merely checking tasks, Bled checked, hurry , Vintgar checked, walk faster...etc. You need at least 3 hours to walk around the Bled lake (6 km i think) and even if you dont go all around you should at least go to cafe where Tito used to watch lake from (near his villa) (fantastic view) and try the famous cake. Vintgar is a couple km away it is 1,6 km (1 mile) long but I didn't go back to car park the same way. I went through the woods and meadows past by a piqtoresque church. That takes at least 3 hours (if you just visit Gorge and go back the same way maybe a little shorter). Bled to Bohinj is maybe half hour drive. Also near Bohinj is beautiful Savica waterfall (slightly uphill) (not in film) which is also worth a visit. Just an opinion.

we DID have a car for our Slovenia portion ... (48 hours) this is what we did:
arrived after dark at Mulej Bled Turist farm.....about a 1.5 miles from lake. I highly recommend the farm...gorgeous, quiet, great breakfast. I do NOT recommend driving after dark in Slovenia (we are in our 50s) signage does not seem to be plentiful, and hey, its in Slovene (?) and the roads were very twisty......it was very white knuckle and frustrating.

after the aforementioned great breakfast, we walked down to the town, rode the plentka boat across the lake to the church. we did not pay to enter the church, but the boat trip is not expensive, and a unique craft of 20? tourists propelled by one well muscled charming Slovenian.

after riding boat back to shore...we kept walking around the lake...gelato in hand, and walked up the steep hill to the castle. (driving up to the castle is also an option, btw). castle has museum....I'm a little bit a museum snob, and I give that museum a grade of B, not B+. ((this is Bled Castle, btw, not the one you were asking about, prejama)) anyway, ticket to castle gives you a discount at either the food court or the fancy restaurant on castle grounds . we decided to spring for the fancy restaurant (not prohibitive at lunch time), ate on the terrace, and this was literally my favorite meal of the trip, including some mighty good food in Italy. AMAZING view down to the lake and its church/island......and a darn good meal. then we walked back down the hill (did I mention that driving up to castle might be a better choice?).....bought some fresh fruit from a vendor, later ate the famous Crème cake at a little café with stout coffee, and walked back out to our Turist farm. (we ended up having a bottle of wine and almonds from the suitcase for supper) fabulous day, but an exhausting 10 mile walk, up hill both ways, as they used to say about the walk to country school.
we left the next morning, stopped in Ljubljana, and just did some puttering. no caves. we also stopped at aquileia to see the mosaics on our way back into Italy.

We're even older than you, and we visited Lake Bled as a day trip from Ljubljana. It's easy to do and saves the hassle of hotel changes. We found the town of Bled underwhelming, but we really enjoyed hiking the Vintgar Gorge.