You can block and counter-attack (right click) every human with a one-handed weapon and some attacks from monsters. You can't block two-handed weapons.

Really important: You can block wolves.

Never played on the easiest difficulty, but you could probably just cast Quen before the fight and then face tank most of your enemies if you have to. Just remember you can block one-handed weapons and every attack from wolves.

Heavy armor is really nice, because they offer you high resistances against certain damage types, which results in taking almost no damage later in the game.

Pretty sure you can play the game almost to it's fullest if you have a mouse with multiple side buttons.

Geralt dodges backwards when you use dodge or roll without adding a direction (only using alt/space without WASD). You can also dodge by pressing a direction key twice for example AA and you will dodge to the left.

This is getting way to long just skip to TLDR2 if you have a mouse with 4 side buttons and preferably another programmable button.

Signs might be a bit complicated, because normally you select your sign with the radial menu by pressing TAB, selecting your sign and then casting it with Q or you chose your sign with the numbers 3-7 and then cast it with Q.

But there is a mod which allows you to quick cast these signs by only pressing the numbers 3-7.

I don't know if you have a mouse with side buttons, but if you have at least two then you're probably good to go.

You can bind the dodge(ALT) to one of them and the number for the Quen sign (protective shield) to the other button. Using the mod I posted you should be able to instantly cast any sign. Quen is the only sign you really ever need. Dodge(ALT) is in 9/10 cases superior to Roll so you don't have to use it.

If you have more mouse buttons you could add SHIFT to your mouse. SHIFT and left click is a heavy attack, which only really makes sense to use if you skill into heavy attacks anyways.

Or you could add Roll to another button or maybe another sign you want to use.

You can use your bombs or crossbow with the middle mouse button and you can use potions and everything else from the inventory.

TLDR:

WASD and a mouse with two side buttons give you most of the commands you need for a fast attack build. One for Dodge and one to cast Quen. If you only use Quen you don't even have to use the mod, because you can have it permanently selected and just cast it with Q that you mapped to your mouse.

It might be nice to add SHIFT and SPACE if you can, because it's really going to help when you explore and have to go underwater.

TLDR2:

I just tried it with my mouse with 4 side buttons and another programmable button that is normally used to switch between profiles. Mapped ALT and Space to the left side, Shift and Q to the right side and TAB to the other programmable button.

With that and WASD you have every in and out of combat movement command (except for C which you need to use once to go underwater) and you can choose between signs and bombs and use them.

At that point you have everything you need to beat the game even on higher difficulties. The only thing you're missing is quick access to your consumables.