1) The schematic is using BT1308W as solenoid drivers. Some schematics still have MAC97 in it. Why did you change to the new ones? Performance?
BT1308W is a surface mount part, and MAC97 is a through-hole part (maybe MAC97 also has SMD version, but not that I am aware of). Their electrical specs are almost the same. I apologize that the schematics are not all consistent. Basically if it’s surface mount you should use BT1308W, and if it’s through-hole, use MAC97.
2) Are the RN2 and RN3 resistor networks only resistors to limit the current or is the ‘isolated’ somehow important? Some talk about opto ccouplers for the triacs or is that just necessary if we would want to switch 220V like main power ?
These are current-limiting resistors. Like transistors, triacs operate under a certain gate current. You need the current limiting resistors to make sure the gate current does not exceed the operating range. These are not for isolation — for that you need to use SSR opto-couplers, such as MOC3063 in conjunction with BT1308W. Even though BT1308W can handle up to 600V, if you want to switch power line devices like 220V device, I would recommend using a relay.
3) I am sure it works fine (I ordered some parts, but could not start assembling anything yet) but isn’t the LM2596 ment to be DC/DC and not 24VAC to DC
You are right, LM2596 is DC/DC switching regulator. The AC to DC rectification is done using diode D1 (S1A) and input capacitor C1 (100uF). These will rectify AC to DC. For additional details, please check my blog post here about 24VAC to 5VDC conversion:
http://rayshobby.net/?p=6002