Intercity transportation in Cuba

There are other ways to get around Cuba and it helps if you speak Spanish. If you spend some time in Havana you might be able to meet a trustworthy driver (or be introduced to one by the owner if you stay at a Casa particular) and you could strike a deal for them to drive you wherever you want.

I traveled by train when I was there, and it was an amazing experience and extremely cheap but there are problems with that mode. Timetable information is almost impossible to find and the process for buying tickets is also obscure. I waited for 6 hours past the departure time in a queue at Havana for the train to Santa Clara and we stopped for no obvious reason at various times throughout the trip.Even still, when I arrived there at 4am there was a line of (horse drawn) taxis waiting to take passengers away.

I also travelled on Viazul and found the train to be a much better overall experience.

If it's not among Viazul stops then Viazul does not go there, simple as a pie!

Viazul is an intercity bus, and that's a key point. It doesn't cover small cities and POIs.
If you want to visit Soroa, you'd better take taxi collectivo or simple taxi. You may not find taxi collectivo if no other people would go to Soroa and you'll have to pay for taxi personally which could be rather pricey.

Last year I went Havana - Vinales - Cienfuegos - Trinidad - Santiago de Cuba - Baracoa with Viazul, and Baracoa - Havana with taxi colectivo.

ad 1. Viazul generally stops only in some restaurants in middle of nowhere. When I was travelling with Viazul there were some people leaving the bus between stops, they confirmed it with driver before. But I don't think it's 100% sure you'll be able to do it. And I also couldn't find official route, so that may not be the best idea. Also, inside it was always super cold (outside was > 30 C, inside people were covered with blankets, it was like 14 C all the time). There were no nightly buses, only departure of 12h long route was at 8 AM, so you're wasting whole day for travel.

ad 3. I don't have it with me but Lonely Planet guide had a list of departure times. The only problem is they were often outdated, the only 100% way to confirm was going to bus station after arriving to some place and asking about departure times, planning your route one step ahead only.

Taxi colectivos are more flexible, but generally more expensive (unless you have more people travelling with you, with 3-4 people it's equal to Viazul). You can find guys providing their services on every bus station. You can set a schedule with them usually. If you're alone you should be able to join other tourists (but with restricted internet access it's often difficult), I just met some Polish guys on the street and organised the route with them.

Just as an anecdote, I had some bad experience with colectivos, me and friends were supposed to go by new car with two drivers switching (Baracoa - Havana is a long way), but we got some old Chinese car with engine that was constantly dying, only one aggressive driver, who at first was driving like crazy, and then started falling asleep behind the wheel. We had to talk to him for some time to keep him awake, after few hours my friend took the wheel and driver went to sleep in the back row, it was too dangerous for him to drive. I'm not saying it's always like this, but you can never be sure, colectivos are not controlled by anyone.

If you'll be sleeping in casas particulares, you can ask the owners for recommendation.

There is also a train but I wasn't able to find any good information about it.

About Soroa, I found this on Wikivoyage

There’s no Viazul buses stopping in Soroa, however since the village starts at the highway junction and there’s an easy stop at the gas station, it's probably ok for the driver to drop you off there. Check with the driver first and then buy your ticket to Pinar del Rio (which is the next stop). The same hold when coming from Las Terrazas, although from there you can also take a cab for 15 CUC. Alternatively take a collectivo from Havana. You’ll find drivers around the Viazul bus station there, but may need to pay the full price to Viñales (15 CUC per person).