Firstly, welcome to our game once again. I've been checking in on your progress closely and you've been doing an excellent job on the role play and the learning front.

Thieves are not always easy to survive with, but they do have some skills which are beneficial to exploration specifically.

So the question is - can a thief be good in defense?
They are a class that I would consider to be extremely defensive, but they can be decent. A skilled thief can be annoying to most classes they are moderate tanks, and they tend to work best with allies.

What skills should I consider if I want to survive in PK?
Acrobatics is probably a must. It will allow you to avoid many forms of lag, which in turn, will let you flee easily.
Shield block will make you more defensive.
Gentle walk will let you avoid aggressive mobs in some circumstances.
The lockpick skills will let you open doors.

Is shield block here as great as it is in some other MUDs?
Not sure how great shield block is on other MUDs. It is behind parry and dodge (for the most part) in its defensive capabilities, but it is certainly a substantial addition to defensive capabilities.

And would race would you recommend? Maybe an arial or a human?
Tough to say, a lot will be based on preference. Arials have supreme dexterity and inherent flight which will help you avoid tripping opponents, but have lowish hitpoints and a nasty vuln (lightning). They will be the best race in terms of raw tanking power.

I think Arial, Human, or Half-Drows would probably make good characters. Duergars and Felars are both more rugged, for the most part, but have more significant downsides.

And of course, having a neutral alignment thief would help you find friends and avoid enemies.