So the plantar surface of 3 toes of each foot touching the ground is wajib, and the toes facing qiblah is Sunnah?
"should" also indicates obligation / duty. Please refer http://www.thefreedictionary.com/should nevertheless, "must" would be more appropriate for a "wajib". I didn't quite grasp what you meant; please clarify (because I did not write "Should bend toes").
"Should" and "Must" are two different things. Again is it not "Should bend toes" i.e not Wajib and "must" touch the ground
From Fatawa Ridawiyyah - 7. Ala Hazrat makes it clear that the "Baatin" or the "Payt" (the hidden or under part) should be placed on the ground. He further explains that it is sunnah to keep the toes are in the direction of the Qiblah. This can happen only if the toes' under-parts are pressed on the ground Therefore it is not just the "tip" of the toes. Also, if someone had long nails and only the nails touch the ground, then toes would not be touching the ground, nor would they be in the direction of the Qiblah.
Fatawa Ridawiyyah 7/376: http://www.alahazratnetwork.org/modules/booksofalahazrat/item.php?page=365&itemid=23 The discussion on the topic of bending the toes in sajda starts from the beginning of that fatwa on page 363.
isn't bending Sunnah Muakkadah, but touching of three toes on the ground Wajib? If bending itself is not wajib then the touching of the tips would be sufficient? Can you provide references if possible please