Thanks for the tips, Craig! I started just going with the assumption that I can't block enemies that have silver in their life bar, but I have no idea if that is accurate, but it's held up so far. I find that I tend to instinctively dodge attacks anyway, rather than try to block. I actually need to remind myself I have a block button sometimes. Yeah, I'm definitely trying to get used to all the skills and wish there was a quicker method of changing them in combat, especially as you want the protection spell a lot (or at least I do), but then may want to throw out fire or mind trick. But I understand the limitations of a controller.
Take it easy, man!
I'm actually playing The Witcher III: Wild Hunt (Game of the Year Edition) on PC with my PS4 controller. I find that I don't really like third person action games with keyboard and mouse. First person, for sure, third person? Not so much.
You are correct. If an enemy's life bar is silver, that means they're not only a monster who can only be hurt by a silver sword, but their attacks are unblockable. Some non-monster enemies, such as dogs, wolves, bears and boars, while they have red life bars (and thus, you'll be using a steel sword to cut them down), they also cannot be blocked/parried.
Just assume this - only humanoid enemies with swords, clubs/maces can be blocked/parried (including enemies who carry shields). Humanoids with large axes, hammers/mauls etc. cannot be blocked/parried either.
As for Signs, I agree. It'd be nice to have an instant hot-swap button of sorts. I'm unsure if this is available on PC via keyboard, I've never actually tried (edit/update: yes, keyboard has hotkeys for insta-use of Signs). I honestly don't use a lot of Signs, personally, but I guess it really depends on your play style. Here's what I do use regularly though:
Quen: I have the 'exploding shield' upgrade, meaning when my Quen shield is done, it creates a small explosion that can push enemies back and even knock humanoids down. I also have an upgrade to Quen that creates a barrier, where if I get struck while the barrier is up, my stamina drains but in exchange, my health goes up. Good way to heal and recover.
Axii: Not only is this used in dialog to get what you want (Witcher Mind Trick, basically) but it can be used in combat to stun most humanoids, and slow down all other enemies. Like I mentioned before, Axii can be used to confuse horses and force them to buck their rider off, allowing you to approach the fallen rider and insta-kill them when prompted.
Yrden: This skill will slow enemies down who walk into the Yrden ring you create. Useful for almost all types of enemies. Slows down fast enemies and makes large, slower enemies even more slower. Yrden is essential to use on spectre type enemies, like ghosts and wraiths.
Aard: I barely use Aard, but it does have its uses. You can use Aard to knock riders off their horses, allowing you to insta-kill them. Aard can also be used to knock flying enemies down from the air, provided they are close enough. Some flying enemies (like harpies) can be insta-killed when they're on the ground. Aard is also used to break down certain walls in dungeons, caves etc. Thankfully, you do not need to level up Aard to achieve any of what I said here.
Igni: Honestly... I think Igni is useless. Don't get me wrong though, I've heard it can be very beneficial, especially in regards to its later upgrades where you can burn away enemy shields but personally, I never use it. The only time I use Igni is when the game requires you to, e.g. solving puzzles, navigation etc.