What this expression means?
When you wrap the 30AWG wire around a resistor, the resistor is a secure core which the thin 30AWG wire is supported.
You can then easily manipulate or hold the component in your fingers while soldering.
Or just solder a 30AWG length of wire (say 10mm) between two stripboard pads 
instead of the resistor?
Do you mean like this? (assuming the wire in the pic is 30AWG):
the upper side of the wire as if it connected to Vcc and the lower part as if it connected to D1 Cathode ?
Now I see what you mean.. the resistor is only for its body.. the current will pass through the 30AWG wire so the resistor has no effect on the circuit..
Yes
Yes
why is preferred to have 1.5A fuse for each solenoid instead of 15A fuse is series with Vcc of PSU?
Does DC jack can handle ~15A of current or I should use this type of connector for the power?
A 15A fuse opens at 15A (usually more than it’s actual rating).
A 1.5A opens at 1.5A (probably 2A).
It makes sense it’s better to protect circuitry at lower currents. 
A terminal strip would work but this looks okay:
is there any DC jacks that can handle more then 2A of current? I might think of deviding my 0 solenoids 1 board into 3 board if 3 solenoids each so only 1.5A * 3 = 4.5A drew on each board so jack should be abke to pass up to 5A
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I'm looking at this dc jack data sheet but there is not current rating
for connection between optocoupler collector and emmiter and R1 2k2 and Mosfet gate - can I use 3.5mm audio jack?
I am just try to think about the most "modular" connection between the two boards.. either audio jack or dc jack will be most modular and easy to mount?
Those are rated at around 500 to 750mA
If you don’t mind crimping your own, maybe JST type connectors may be the way to go.
However, these are not chassis mounted.
you mean the dc jacks I shared?
so they are not good for the PSU power connection but I can use it for the 15 meter two conductor wires between optocoupler and mosfet board?
Yes
- BTW, link us to the solenoids and motors you are using.
those are the motors I'm using.
regarding the solenoids - It either will be those that rated for 800mA (0730B 12V) or those rated for 300mA
I'm not sure what to choose.. those motors should knock mainly on small glass objects, wood objects and maybe some aluminum object ( see example in video link)
btw - the video attached I used those solenoids but I think they were too weak as something they got no force to go back to their OFF position after being ON
Can I use DC jack for running the 10 - 15 meter long wires between optocoupler board and mosfet board?
Read this.
Leo..
If my solenoid is rated for 12V 1.5A its means that the internal resistance is 8R, right?
If using 9V instead of 12V its means that the current pass through the coil will be 1.125A ?
Is it just means that the solenoid stroke will be weaker? but will still be functioning ?