Let me start off by saying that Curse of Strahd certainly has some very challenging encounters, many of which can be stumbled into at points when the party is woefully under-levelled. I've only run it once so far, but that did end in a TPK.
In Vallaki, the party has the opportunity to run into a nest of vampire spawn whilst investigating a coffin maker's shop. The coffin maker (at least under my interpretation) is being kept as an unwilling servant/hostage of these vampires. I tried to give as many hints as possible that the coffin maker was scared of something in the shop - he was glancing over his shoulder, refused to let them in, would only speak in a whisper and asked the party to meet him later 'somewhere safe' suggesting the inn. Unfortunately the party didn't take the hint. Instead, they separated, two distracted the coffin maker, whilst the remainder broke into his house. I tried to give further hints - letting them spot many footsteps around large body sized crates in the dust of the storage room. I even pointed out that the cleric in the party felt something deeply disturbing from the room. Irrespective, he cracked open the crates, triggered an enormous fight and ended up dying.
It's important to note, however, that this type of danger is a common feature of sandbox campaigns. On top of that, this is a horror campaign - it is not unreasonable that if the characters poke around the dark corners of Barovia whilst uninformed of what lies within, they may end up angering something much more powerful than themselves. I personally feel that without such a threat, the player paranoia and tension necessary to maintain an effective horror theme can end up being lost or diminished.
Players should be made aware of this when starting the campaign and they should note that often discretion will be the better part of valor. It also behooves the DM to provide more narrative warning in such cases - signs of danger, direct warnings from NPCs, and (for when it all goes awry) avenues of escape. Even with this, however, encounters can and should have the potential to be fatal. At that point it is down to each DM how such an encounter plays out - nothing precludes you from granting strokes of luck, fudging rolls, or throwing in timely ally arrivals when things go south. Only you know your players and the game you are running, hence it's up to you and what you most feel makes for a fun game around your table.
With regards to your specific examples:
Death House - this is a hard starting adventure, but one that I found to really set the scene and vibe of the campaign. The shadows at the statue, the two ghasts, and the shambling mound stand out as all potentially being fatal encounters. The shadows, however, are extremely avoidable and are essentially a consequence of a lack of caution. Much of their threat depends on the party composition and their ability to output magical damage. Contrary to the book I would probably not have the shadows pursue the party out of the room, and may have them vanish if (for example) the orb was returned. The ghasts and the mound are tough, but certainly doable if the characters play smart.
and
Bonegrinder - I feel that warnings discouraging the party from entering too early are key to this adventure. The Vistani at Tser Pool may have warned the party away from there, knowing their destination and based upon what is written on pg. 28. Additionally, the raven should be extremely vocal in trying to ward off the players. Remember, the raven will be in league with the wereravens of Vallaki and essentially a force of good. Hence, assuming the party did head in and end up in fight against the three hags, having the raven summon help in the form of a swarm or a wereraven may be one solution to avoiding a TPK.
Having studied the book, you have some idea of what each Tarokka result can entail, and you can let this "foreknowledge" inform Madam Eva's reactions to the cards as they appear. If you draw
the 7 of Coins, you can hint that the location of the treasure isn't very far away; if you draw the 9 of Coins, Madam Eva might shake her head despondently and say "Maybe save this one for last."
It's easy to want to add merely emotional clues, to add color without giving too much away: "It is in a place of dread;" "I feel the presence of mournful spirits near this item;" "This place is cold and silent." But there's so little emotional contrast in Barovia that sentences like these can easily fade into the background noise of doom and gloom. Specific information will make your players' ears perk up, and if your goal is to create a memorable experience, you can afford to drop a few extra hints.
For example: Many of the speeches for cards that clue
locations in Castle Ravenloft don't actually refer to the castle ("We know it's in a crypt. Do we know where a crypt is?"), but Eva can add something about "beneath the Devil's tower" or glance nervously in its direction.
If the players feel like gabbing it up with Madam Eva about each card, trying to pry hints out of her, go ahead and respond according to the character you've built in your performance—but to add emotional texture to the scene, make her insist on total silence before she turns over the next card.
The use of a real deck is so effective because everyone loves props. The description of Madam Eva's tent provides a lot of inspiration for additional props: Mysterious lights, a crystal ball, and a black velvet cloth. I think this cloth is a good thing to focus on because you can slowly unroll it before you lay the cards down, which is a very evocative and almost ritualistic action. Even if your players forget what cards you drew, they will remember the appearance of that cloth unrolling across the table.
If you can't get a piece of black velvet, use whatever spooky-looking fabric you can get your hands on, and then change the words "black velvet" to whatever you're using when you read the description, so that the players will think you are following the details in the book exactly.
(On the subject of
Strahd's Enemy: Some of the results are more interesting than others. The book gives you the choice between NPC A and NPC B for some results, apparently according to what kind of Challenge Rating you want for the party's ally. Obviously you want to figure out your choices in this regard ahead of time. But for my purposes, I didn't really want my players to end up with either version of the Broken One, the Horseman, or the Innocent, so I just removed those cards from the deck. If you're not comfortable with the full range of randomness in the book, you can create "massaged randomness" without the players finding out.)
Best Answer
Prep one session around your players
After a few sessions of LMoP, you will have a good idea of how far your party can get in a day. Even though CoS is somewhat sandboxy, there's no way they're going to end up in
in the first session. Travel takes time as well, and you can throw some combat encounters at them along the way to cities that you haven't fully prepped to finish out the rest of a session. Both inside and outside the cities, there's a hierarchy of things that do not need to be fully ready to go as soon as the characters reach the city.
Choose your session end point carefully
If your players are in a particular city with multiple things to do and see, and they choose a path you didn't anticipate, leave them on a cliffhanger. Obviously this doesn't work if you literally just sat down, but if you choose your end point such that a particular action on their part is unresolved, you know what they are going to do first next time. End the session when they, exhausted after a rough day of travel, finally see the gates of the next location. You then know that they will need an intro to a particular city at the start of the next session.
Steal
Many other DM's have run CoS and written it up, on blogs of their own, reddit, and here. They've all fleshed various things out, emphasized the bits they or their groups found important or compelling, and can help direct your prep a little.
Withhold information from your players
Don't give the players a reason to leave
before you've fully prepared the next area. You should know where they are likely to go next, based on the hooks you've provided, and maybe all their investigative efforts until you've prepped what comes next point to things inside the city, or the questgivers could indicate that what they are asking for is a very dangerous endeavor. Or,
You're the DM. There are so many tools at your disposal to keep them away from parts of the adventure you're not ready for them to experience. Remember,