Those mechanisms have more than one degree of freedom thus we cannot find the instant center of rotation for some of the bars, in that case, we cannot describe their movement because we do not know where is such point. The movement of 1 degree of freedom mechanisms can be described with precision, for example, the movement of the mass m1. Great article, greetings! @acont
Thanks for your valuable contribution @acont it is great to find people like you with great analytical skills. A brotherly greeting, my friend.