Where and when did Tantra start, if it had an origin?
While there are many opinions, there is no real beginning as such. See articles and books, particularly the Hindu vedas, and the spiritual philosophy of Tilopa and Milaropa. However, it is now known that using sexual-heart energy of the Kundalini for transcendence and connecting with God or Divinity was known by many cultures throughout the world. There are references to it in both the new and old testament's of the Bible, in the Koran, the Bhagavagita and countless other spiritual texts.
Are there certain types of cultures where it arises?
Interesting question. Tantra seems to arise and gain interest when we have gone too far with our material world. More of the lay-back, natural cultures like the Cherokees and Polynesians follow a similar lifestyle. Some pagan traditions also do. Whenever civilization has filled its members with fear, Tantra becomes popular. It is something we all experience at one time or another in our lives. Now, in Russia, Tantra has become very popular.
Are there historical or fictional people associated with its origins?
You can see glimpses of Tantra in Krishna's playfulness with the dakinis, and Jesus, no doubt, was also a Tantrika based upon his words of love and his association with Mary of Magdalene and Veronica of Nazareth. Tilopa, Naropa and Milaropa also were the so-called original messengers of Classical Tantra. However, even with the writings of Sir Richard Burton, much has been lost to antiquity
Is there an 'ideal' Tantra, regardless of how it manifests today? If so, what is this like?
Returning to the Biblical Garden of Eden, or a vision of Nirvana or Heaven. A community of people filled with unconditional love, bliss, a joy of life, tenderness and acceptance of all. Enjoying the good life, so to speak, unaddicted yet experiencing with totality and enlightened, of course [chuckle].
Is there an objective or ideal focus of a Tantrika? If so, what is it?
Not really. Maybe enjoying God's existence at every moment and allowing enlightenment to occur in the process. Becoming aware...awake. Follow a form of mantra or yantra meditation, eat a proper vegetarian diet, be gentle and find love within you at each moment. The lifestyle itself is the gift and path....
What are the essentials of 'Tantric philosophy' if there is such a thing?
Same as above.
How does this relate to other Eastern or Western philosophies?
Most philosophies and religions have a dogmatic point-of-view of existence. Eastern and Western philosophies are tied up in dogma. Tantra is about dropping dogma, and simply being in bliss. This is difficult to talk about since all talk is the beginning of a philosophy. Philo from the Greek language means "life" and "sophy" from sophic represents study. In Tantra life in its totality is beyond study... It is experience.