- Let’s say I do not have any items, if I spend 71 points into that tree, what will my critical chance be?

Sorry, i am going to ignore that tree right now because i don't have the game opened and the tree in itself is noting that needs to be addressed specifically and i dont know much about the sorcerors skill trees. I would say just test it, make a new sorceror, use tab+D to open dev mode, level it to 100 and fill the tree with points.

- If I end up for example with 30% base critical chance(from the tree) and have three items and each adds +30% to critical chance. Do I end up with 65,91%?

I think you will end up with 30 * (0.30+0.30+0.30) = 57%

- Will there be items that give flat critical chance?

Only weapons. Some have it as a white stat. (max 8%) and they can have it as a blue stat (3,4,5,6,7% respectively for level 1-5)

As you indicated, there is 5% base crit chance (and 200% dmg on crit)

Now there are 3 modifiers for crit.

-Added crit chance. This is added to that 5%.

-Increased crit chance. This is a multiplier on the total you got after your added crit chance.

-Increased crit damage. This is added to the 200% multiplier.