My understanding and experience from holding both memberships:
Accor limitless is membership based on amount spent and they track it in form of status nights and status points . It cannot be purchased right away after silver membership
Accor Plus has traveler and explorer membership which can be purchased and get us extra perks and discounts . It also gives silver limitless membership automatically
Sharing my understanding
1) Can we book any type of hotel in accor limitless?
Any room which is available for booking on accor app and website. Some hotels have some rooms which they may choose not to open to Accor site based on other tie ups etc
2) Can we pay via accor points at any hotel and for non refundable rooms also?
Non refundable rooms : For most non refundable rooms Accor site gives an option to pay by points while booking but the rate per night increases slightly. If it's not giving that option either we have less than 2000 points or point category not available (very rarely seen)
Refundable rooms: Yes most of the times it's a breeze to pay by points when doing the check in itself or at check out , just takes a little time as hotel staff may have to call a manager or other staff person who knows how to redeem points