“Contract with the Abyss” is used as a generic substitute for Ritual Spell Cards like “Black Illusion Ritual” and “Contract with the Dark Master”. You cannot use “Contract with the Abyss” to Ritual Summon a non-Ritual Monster like “Dark Magician”.
“Cost Down” will not reduce the number of Level Stars needed to perform a Ritual Summon, even if used with “Contract with the Abyss”.
If you Ritual Summon a Ritual Monster using “Contract with the Abyss”, it is considered properly Summoned, so you can use “Monster Reborn” on it afterwards.
Netrep Q&As:
Q: “Contract with the Abyss” states in its text that the Level Stars of the monsters being offered as a Tribute equal that of the Ritual Monster being Ritual Summoned. Can the total of Level Stars being offered be higher than the Ritual Monster or must it be exactly equal?
A: The total Level Stars of the monster(s) offered as a Tribute for the Ritual Summon using “Contract with the Abyss” must equal the Level Stars of the DARK Ritual Monster you are attempting to Ritual Summon. You cannot go above the Level Stars of the DARK Ritual Monster. So if I want to Ritual Summon “Dark Master - Zorc” using “Contract with Abyss,” I must offer monsters whose total Level Stars = 8. I cannot go above 8. I could offer two 4-Level Star monsters and the total would equal 8. I could offer four 2-Level Star monsters, because they would equal 8. I would not be able to offer a 5-Level Star and a 4-Level Star monster because their total Level Stars would equal 9. Curtis Schultz, 02/23/2004