I'll give an overall point of advice about all channeling first, and say that if you don't have an ability that specifically says you can do a particular thing, then it's likely you can't do that particular thing. This applies to all types of channeling, not just sorcery. Channeling in this world is generally restricted to specific patterns, forms, and techniques that have been developed and practiced over time to do a particular thing. Being able to simply freeform manipulate what the channeler wants based on a mental idea would be considered the elusively top-tier stuff, like Nuumic masters of elemancy or the highest echelon of Rook Parlour or Rhuidim sorcerers. That is to say - this is the stuff that's generally reserved for lore/historic figures and rare NPCs. Part of this is the lore, and part of it is that because this is an open, anyone can play any time, no GMs required, game. If this were pen and paper with people around the table, the DM could effectively guide and moderate just what the channelers could and couldn't do with their powers. With a multiplayer no-supervision-required MUD, there's no way to watch, guide, and moderate all the players wanting to freeform do whatever with their channeling. And for better or worse, the magic in this world is more restrictive and tricky to use - particularly in new, non-painstakingly-established ways.
1. Can I shape the nether according to what I wish it to look like. I.E. If I can create a blob of nether that floats around me, can I then will it to take another shape once it has been formed? If this is true, then how much control do I have in moving it about? What level of sorcery and channeling would reflect the ability to do this sort of witchcraft?
Conjurations can be tricky. Even with nether, being the easiest type of channeling to conjure up actual "stuff" with, and able to do a surprising variety of things beyond what one might initially expect (be cold and deconstructive to other matter). But getting channeled conjurations of any kind to do a lot beyond showing up and guiding their location around is difficult. There have been some techniques developed to do certain things - which player characters can learn as "abilities" from their guilds - but those are established rote techniques that are taught because they work reliably.
So, channelers can't just say "Okay conjuration, take the shape of a rose." First off - communicating the exact details of your will to the conjured matter isn't a simple or perfect process (though generally easiest to achieve with nether vs other conjurations). A general "go that way" command is easy enough (it usually seems to have a harder time "listening" once taking the initial direction), but how to give the fine details of forming a very specific and very detailed form or image? How do you actually communicate that, and instruct the conjuration to apply it? It's more than just picturing it in your mind and saying "do that".
In my own circumstances, I think of it like the ability to draw a realistic picture. I know what a rose is. I know what it looks like. I can picture exactly how I want it in my mind. But if I put a pencil to paper, it's not going to come out well. Now, I'm not saying shaping channeled conjurations is specifically a matter of artistic skill or talent. This example just works for me, because I'm terrible at drawing - I'm trying to use it to get the idea across that just because you can visualize or know what you want something to look like doesn't mean you can apply it simply or effectively to what you're trying to create. I know in my mind exactly how I want the rose to look, but I try to draw it and it looks like a distracted toddler tried and failed to draw a fire flower from Super Mario Bros (the first one). Some NPCs might display the ability to make impressive displays with conjurations, but I've never subscribed to the idea that "if any NPC in the world's history can do it, player characters should be able to do it". Either that NPC has studied or developed an uncommon technique that isn't available to player characters, or - far more rarely - they have an extraordinarily rare talent that allows them to pull this off freeform.
So the first issue with your proposed scenarios is that no, most sorcerers can't effectively shape nether into detailed forms as a simple act of will on the spot. But let's say for sake of argument that we're talking about one of those rare cases who can. Now we've got the trickiness of nether to contend with. It's often described as "amorphous" or "nebulous" and such. This is because it tends to not want to sit still or keep the same form. In its "natural" state - or rather its non-conjured form when not being controlled by a human - it seems to copycat a lot of things. Humanoid forms, spider forms, even plants. This is nether seemingly acting of its own accord, however. It's not taking on those shapes because a sorcerer is standing nearby making it so. Maybe it's because sorcerous channeling is an imperfect means of communicating a sorcerer's desire to nether, maybe it's because nether needs to be in a particular state first to have a more lasting specific form, maybe it's because nether is somehow mischievous or rebellious and actively resists being controlled once conjured beyond a few simple commands like "go that direction" or thoroughly-practiced and established processes like the sorcerous abilities player characters can take. Whatever the case, beyond a few special circumstances and established techniques, it's not easy to keep nether to a task for long.
2. Can a formation of nether be manipulated by physical means. That is to say, if I channel a mass of nether in front of me, and play an instrument, can I allow it to take the form of the sound wave. I imagine since it can be blocked with a shield, that there's some degree of opposing force being issued on it, but I'm unsure if I have influence over what can or can't affect it.
See above - I don't think there's any way to effectively communicate to the nether, "take the form of this sound wave that's about to hit you." And what would nether taking the form of a sound wave ... be?
But nether -is- affected by physical things, in case that was in doubt. It can be blocked with shields, as you mentioned. It doesn't just vwoop through solid matter - a nether-wraith couldn't just float through a closed wooden door or a glass window pane, for instance. Swing a branch at a phantom and the nether your branch swings through will swirl and eddy a bit in response, being disturbed by the branch passing through it. Nether certainly appears to occupy physical space - at least, most of the time.
3. Since inorganic materials can become tainted, is it possible for something like snow to become tainted? What happens if it melts, or is ingested? Does it turn black?
Nether can indeed taint inorganic matter, often causing the matter to appear blackened or whorled or mottled with dark stains. How and why this happens isn't common knowledge. Why doesn't the nether damage the matter like usual? Is it "inside" the matter, has it corrupted and converted the matter into something else, is it some weird "the matter is two things at once now" phenomenon? I can't think of any examples of nether-tainted snow, but are you familiar with voidsalt?