So my single biggest irk in Gothic 1 right now is that many enemies LOVE to make some kind of jumping back animation, which makes them become temporarily invincible. I attack, I CLEARLY hear my weapon hit, but because they did the jump-back animation they take no damage.

Is there ANY good way around this, or does it really function on an enemy by enemy basis?

Pretty annoying, isn't it? :-)
Some blows (the critical ones, I suppose) make damage even if the enemy is dodging.
Anyway you could force yourself in defense mode (CTRL+DOWN) and let the enemy charge you: you will not be hurt and could efficiently strike back.

I know what you mean, but I concluded that it must have been left there, unintended, otherwise swinging your weapon in vain wouldn't have made a proper hit attack sound.
The only solution to this, is to work calmly on your attack timing - meaning don't swing the weapon as soon as the enemy gets close. Wait for THAT fraction of a second before hitting the enemy, they won't hit you anyway, although it sure looks like it.

Also, on a side note, when case, you could try corner the enemies against a wall, so they won't have space to back up indefinitely.

I've actually found myself almost never blocking attacks in G1. It only seems to activate at certain times (not while I'm being attacked) and then it only lasts for a second or two. Not a bad idea though, I'll probably have to play with it a bit (if there are any enemies left in the game at this point).

The more you learn how to use your weapon, the longer block mode lasts.
Go and become Expert or Master in one handed or two handed weapons (or even both).

Easier put, you do a parry move (or parade) when you press Ctrol + S.

It is a good idea to learn the move, because if you'll look carefully, you'll see that every NPC of the same type uses a standard attack combo, thus you actually know or predict 100% accurately what their next attack will be.

For example: When a humanoid attacks you, you should do the following
- parry (Ctrl+S)
followed by a quick side attack
- side attack (Ctrl +A / D).

This works well when you're rookie. If you master your weapon, you can do:
- side attack (Ctrl+A/D) + power attack (Ctrl +W).

...and repeat.
Just keep in mind that you need to focus on your timing, and that attack speed increases as you raise your mastery with a type of weapon.

P.S. Concerning the orcs, they always open the fight with power attack, so you don't need to worry about parrying it. Just do a side attack (or two) and back up one step. Repeat. Also, this works in G2 as well.