Is the use of "Ellebana" —which increases the density of eyelashes and remains for a period of time— considered to fall under the ruling of al-wasl (joining/extensions)?

It is permissible to use this technique if it does not harm the eye, because the default rule for things is permissibility. It has no connection to the forbidden hair extensions (wasl), as it is merely coloring and lifting the eyelashes, not adding hair. However, if it is proven to be harmful to the eye, then it becomes forbidden, due to the Prophet's (peace and blessings be upon him) saying: "There should be no harm nor reciprocal harm."