Yog-Sothoth

If I pay 2 to trigger Yog Sothoth’s (The
Yuggoth Contrat F99) ability and target a spell
event card in my discard pile that then says:
“attach to a character,” does it still return to the
bottom of my deck (as stated on Yog Sothoth)
or does it stay in play?
The card is returned to the bottom of
the deck, per Yog Sothoth’s ability which
reads, “Action: Pay 2 to choose and play
a Spell event card from your discard
pile without paying its cost. Then, place
that card on the bottom of your deck.”
The subtype or card type do not prevent
the rest of Yog Sothoth’s effect from
resolving.

How does Yog-Sothoth Lord of Time and
Space’s (The Yuggoth Contrat F99) triggered
ability which reads, “Action: Pay 2 to choose
and play a Spell event card from your discard
pile without paying its cost. Then, place that
card on the bottom of your deck” interact with
X cost Spell event cards in the owner’s discard
pile ? Would Yog-Sothoth be able to pay 2 to
choose a copy of Unspeakable Ressurection in
the discard pile to bring a character of any cost
into play from the discard pile?
Yog-Sothoth interacts with X cost spell
event cards in the discard pile by
circumventing the cost of X being paid
entirely. However, because the cost of
X is not paid, X is defined as 0 for the
purposes of card effects that reference it.
For example, if I use Yog-Sothoth’s
triggered ability and pay 2 in order to
play from my discard pile Unspeakable
Ressurection (which costs X), which
reads “Action: Choose a character in
your discard pile with cost X or lower.
Put that character into play.” I can only
put into play a character that costs 0
because I circumvented paying the cost
of the card.

Does Yog's special ability work with Tesla?  I could see it counting only as a triggered effect by Yog and not by the card in the discard pile, but you are playing the spell from the discard pile without paying its cost.  Does this mean you are triggering the spell's effect from the discard pile?