1. If I send professor Nathaniel peaslee to the discard pile through ritual of summoning will I still be able to bring it back
2. If I bring back museum curator back from the discard pile with peaslees effect will I get it's enters Play effect
3. Can I use ritual of summoning effect more than once per turn
Nathaniel Peaslee says: "Action: After Nathaniel Peaslee enters your discard pile from play, pay 2 to put into play all character that entered a discard pile this phase"
Museum Curator says: "Response: After Museum Curator enters play, search the top 5 cards of your deck for a support card and put it into play. Put the rest of the cards on the bottom of your deck."
Ritual of Summoning says: "Action: Sacrifice a character to put a success token on this card" and "Action: Discard 5 success tokens from Ritual of Summoning to put a character into play from your hand."
1: If you used Ritual of Summoning on Nathaniel Peaslee, he would have had to be in play to being with. Thus he would be entering the discard pile from play, thus the answer is yes.
2. The FAQ tells us that "Enters Play" is an all-encompassing term that covers both "Put into play" and "Play". Peaslee will "put into play" the Museum Curator, thus it will "enter play", and thus the ability can trigger.
3. (I'm going to assume you mean the second Action about putting a character into play.) Yes, because there is nothing that prevents it, assuming you have the 5 success tokens.
So, if you have the resources around to do it, you could sacrifice Peaslee and 4 other characters three or four times in a row to bring in cards from your hand. If you also used Itinerate Scholar, you could get an extra round in there by stealing your opponent's domain (if available).