NO. Not in the strict scriptural sense of the word Guru.
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).
Guru or Teacher in the Vedic Dharma:
The first birth is from the mother and the second at the tying of the
Muñja-grass girdle. At the latter birth, the Savitri verse is his
mother, while the teacher is said to be his father.
The man who initiates a person and teaches him the entire Veda is called ‘teacher’; whereas someone who teaches just a section of the
Veda is called ‘tutor’, as also someone who teaches the Vedic
Supplements.
VDS 3.20
So, obviously, the teacher or Guru has to be alive to initiate the student and teach him (the student) the Vedas.
The student, after being initiated, has a lot of duties and he has to stay along with his teacher in his (the teacher's) house.
Student––he should serve his teacher until death and, when the
teacher dies, the sacred fire; for it is stated, ‘The fire is your
teacher’
12 He should wait upon his teacher, walking behind him if he is
walking, standing by if he is seated, and sitting if he is lying down.
13 He should recite his lesson only when he is called upon to do so.
14He should announce everything he has received to the teacher and eat
with his permission.
VDS verses.
So, it is more than obvious now that we are talking about a living Guru here.
Guru in the Agamic Dharma:
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.
EkAkshmarapradAtAram yo gurunchA vamanayet |
SwAnam yoni satam gatvA chandAlatvam vApnuyAt ||
....
One who insults the Guru, who is the giver of ekAkshara (or OM),
gets the womb of dogs for hundred births and thereafter gets chandAla-hood (low human birth).
KT 11.74.
So, here the Guru is defined to be the one who gives disciple the ekAkshara or the OM. Now, obviously doing so is not possible for someone who is dead.
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.
The next verse says, that all kinds of knowledge, conducts, scriptures, mantras are successful only if they are learned from Guru's mouth. And, there is no other way.
PAramparyA AgamAmnAyAm mantrAchArAdikam priye |
Sarvam guru
mukhAllabdham saphalam syAnna chAnyathA ||
............
ParmaparA (lineage), Agamas, AmnAya, Mantras, AchAra (conducts) are
all fruitful if obtained from Guru's mouth ( i.e directly
learned from a personal living Guru), and otherwise not.
KT 11.46.
Further, it is stated in scriptures that just like a wife's sins get attached to her husband, likewise a Guru has to pay for his disciple's misdeeds. Hence, Guru, before initiating the disciple, has to test him in many ways. In fact, they both have to mutually test each other before the DikshA.
The following verse gives the amount time for which such tests are to be conducted:
EkAvedana dvijo yogyah kshatriyo vatsara dvAt |
Vaishyo yogya
stribhirvashairshchaturbhih shudra eva cha ||
...........
The Brahmin becomes competent [for dikshA] after one year, a Kshatriya after two
years, a Vaishya after three years and a Shudra after four years.
KT 14.105.
EkAvedana in the verse means one year. So, for a Brahmin disciple, the mutual test time is one year, for Kshatriya it's two years.... and for a Shudra disciple it's four years.
Now, obviously Guru only if he is alive can conduct such tests and can be the subject of such tests.
Can I have multiple gurus (Guru Tulya) unless I get a guru where I get
personal attention, which is not possible these days?
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 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
And, similar thing has been stated in the Uddhava Gita too.
However, there can be some exceptional kinds of Gurus. Anyone or anything who (which) teaches you something is your Guru.
Lord Dattatreya is known to have 24 such unusual Gurus.
In the Uddhava Gita, a song embedded in the Bhagavata Purana
(11.7-9), Dattatreya tells King Yadu about his 24 unusual gurus:
1. The Earth
The Earth is disrespectfully trodden on by all creatures but bears
them all without complaint. So from Her, I learned to accept all of
life’s pleasures and pains with forbearance.
2. The Wind
Wind is of two types, Prana refers to the internal life forces and
Vayu refers to the external movement of air. Prana is in all bodies
but takes on the shape and form of the particular body it is in. Vayu
passes everywhere but does not remain in one place. So from Prana and
Vayu I learned to avoid attachment to any place but to adapt to
wherever I might find myself.
3. Akasha
Akasha is one and all-pervading. From it I learned that Brahman is one
and all-pervading.
4. Water
Water purifies and sanctifies. From it I learned to be a source of
purity and sanctity.
5. Fire
Fire burns things leaving only their essence. From it I learned that a
yogi should see in beings not all the false opposites like rich-poor,
old-young, high-low, but only the Atma which is the essence.
6. Moon
The one and same moon appears to go through changes, waxing, waning
etc. From Him I learned that the one atman only appears to go through
changes such as birth and death.
Abbreviations used:
KT- KulArnava Tantram ( All verses in this text are spoken by Lord Shiva to his consort Sri Devi). Also, KT 1.100 means verse number 100 of chapter 1 of the text.
VDS- The Dharma Sutras of Vashishta.
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. Doing in that fashion one can complete his DikshA.
