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In both Vedic and Agamic Dharmas, a Guru has to be someone who is alive. I too have some friend who considers Adi Shankara as his Guru.

But unless he takes DikshA from a living Guru he will be considered technically as an uninitiate (adikshita) only. Such mentally accepting someone as one's Guru may look fancy but does not really work. (Having said that in very rare cases, one is allowed to accept Lord Shiva or Mother Goddess as their Guide-Guru but that's an exception not a rule).

The above is the procedure only if Guru (who gave the mantra in dreams) is inaccessible. If the Guru is reachable, then one has to receive the same Mantra from him yet again.

In both Vedic and Agamic Dharmas, a Guru has to be someone who is alive. I too have one friend who considers Adi Shankara as his Guru.

But unless he takes DikshA from a living Guru he will be considered technically as an non initiate (Adikshita) only. Such mentally accepting someone as one's Guru may look fancy but does not really work. (Having said that, in very rare cases, one is allowed to accept Lord Shiva or Mother Goddess as their Guide-Guru but that's an exception not a rule for everyone to follow).

The above is the procedure only if Guru (who gave the mantra in dreams) is inaccessible. If the Guru is reachable, then one has to receive the same Mantra from him yet again. Doing in that fashion one can complete his DikshA.

MadhuluvedhA yathA bhringah pushpAt pushAntaram vrajet |
JyAna luvdha stathA shishyah guro gurvantra vrajet ||

.........

Just like the bee, desirous of honey, moves from one flower to the other, a disciple, desirous of knowledge, can likewise move from one Guru to another.

KT 13.132

Regarding mantras received in dreams, i will probably update the answer later.

MadhuluvedhA yathA bhringah pushpAt pushAntaram vrajet |
JyAna luvdha stathA shishyah guro gurvantara vrajet ||

.........

Just like the bee, desirous of honey, moves from one flower to the other, a disciple, desirous of knowledge, can likewise move from one Guru to another.

KT 13.132

Updating the answer regarding Swapna Lavdha Mantra (mantra received in dreams)

The TantrasAra quotes the following verses:

Swapno lavdho mantro yadi sathgurum prApnoti tadA tata eva tanmantram grihniyAt nochet jalapurna kalase guroh prAnpratishthAm vidhAya vatapatre kumkumena likhitam mantram tat kalase prakshipya uttalya mantram grihniyAdityarthah..

...........

If Sathguru is obtained through a mantra seen during dreams, then one has to receive that mantra once again from that Guru (in reality). However, if that is not possible, then the following procedure is to be adapted- Install the life-energy (prAna) of the Guru in a water filled Kalasa, then write the received Mantra in a Pipal leave with Kumkuma and throw that leaf into the Kalasa. One can then take back the leaf out of the Kalasa and receive the Mantra.

The above is the procedure only if Guru (who gave the mantra in dreams) is inaccessible. If the Guru is reachable, then one has to receive the same Mantra from him yet again.

In both Vedic and Agamic Dharmas, a Guru has to be someone who is alive. I too have some friend who considers Adi Shankara as his Guru. But unless he takes DikshA from a living Guru he will be technically a uninitiate (adikshita) only.

So, obviously, the teacher or Guru has to be alive to initiate the student and teach him (the disciple) the Vedas.

So, it is more than obvious that we are talking about a living Guru here.

In Agamic DikshA SamskAra too, a Guru has to alive in order to transfer energy, consciousness etc onto the disciple. The following verse defines who is the Guru precisely.

DikshA is actually a process where the Guru gives mantra into Shisya's ears a particular number of times. Their nearness (sAmipya) is essential in the process. Needless to say both have to alive to participate.

Now, obviously Guru who is alive can conduct such tests and can be the subject of such tests.

Well, such persons have not actually become your Gurus. So, there is no harm in mentally accepting more than one such Saints as your mentors or protectors.

Anyways, even in case of actual Gurus, more than one Gurus is allowed.

MadhuluvedhA yathA bhringah pushpAt pushAntaram vrajet |
JyAna luvdha stathA shishyah guro gurvantra vrajet ||

.........

Just like the bee desirous of honey moves from one flower to the other, a disciple desirous of knowledge can likewise move from one Guru to another.

KT 13.132

However, there can be some exceptional Gurus. Anyone or anything who (which) teaches you something is your Guru.

In both Vedic and Agamic Dharmas, a Guru has to be someone who is alive. I too have some friend who considers Adi Shankara as his Guru. 

But unless he takes DikshA from a living Guru he will be considered technically as an uninitiate (adikshita) only. Such mentally accepting someone as one's Guru may look fancy but does not really work. (Having said that in very rare cases, one is allowed to accept Lord Shiva or Mother Goddess as their Guide-Guru but that's an exception not a rule).

So, obviously, the teacher or Guru has to be alive to initiate the student and teach him (the student) the Vedas.

So, it is more than obvious now that we are talking about a living Guru here.

In Agamic DikshA SamskAra too, a Guru has to be alive in order to transfer energy, consciousness, mantras etc into the disciple. The following verse defines the Guru more precisely.

DikshA is actually a process where the Guru gives mantra into Shisya's ears by chanting it a particular number of times. Their nearness (sAmippya) is essential in the process. Needless to say, both have to be alive in order to participate.

Now, obviously Guru only if he is alive can conduct such tests and can be the subject of such tests.

Well, such persons have not actually become your "Gurus". So, there is no harm in mentally accepting more than one such Saints as your mentors or protectors.

And, even in case of actual Gurus, having more than one Gurus is allowed ( I myself have two Gurus).

MadhuluvedhA yathA bhringah pushpAt pushAntaram vrajet |
JyAna luvdha stathA shishyah guro gurvantra vrajet ||

.........

Just like the bee, desirous of honey, moves from one flower to the other, a disciple, desirous of knowledge, can likewise move from one Guru to another.

KT 13.132

However, there can be some exceptional kinds of Gurus. Anyone or anything who (which) teaches you something is your Guru.