1. How to win with Necromancers with the patch, given the restrictions on Vampires and Necromancy Enhancement?

Don't play Necromancer. It's obvious. How? It's impossible to explain in a single word. You need to see how others do it. Then you'll understand. (Streams can help)
According to the HotA rules, you can't start with Galthar, and you can't take Isre or Vidomina with Expert Necromancy until all the skills are maxed out. For example, start with Clavius (or Tanta, depending on your mood). On the first day, it's best to build a Skeleton Converter, and convert everything from another hero into skeletons. And carefully start playing as a Necromancer. Your goal is to get Expert Necromancy as quickly as possible. I even give up Earth magic, and Expert Necromancy will be available at level 5, or even level 3. And take Logha if you get the chance.
Then carefully accumulate bones. It's difficult to build a stack at first, but then everything goes well.

2. Why is Soyla so good for Conflux?

Let's call her Ziel? She's a specialist in ranged attacks. If there are very early encounters, you can solve the final battle purely with ranged attacks. She has decent ranged attack. That's what makes her good.
Taking her as a main hero on large maps is a risky undertaking.

3. The patch prevents digging up the Grail until the second month, does this mean that digging is impossible at all? Or can you find it and dig it up in the second month?

I don't know about the patch, I haven't tried it. The Grail is usually not something I'm interested in. But I think you can dig it up in the second month. But you can check that yourself. (Try digging after day 221).
Or someone else who knows will respond.