No. Normally, you name your sins by kind and number. If it’s been a long time, or if you are an adult making your First Confession, you can “ball park” it - you can say, “I did this sin (name the sin) once a week for ten years,” or “about six times a year from the age of 12 to the age of 25” etc.
No. Everyone I know who lived as an unbeliever, also died as an unbeliever, and had their funeral in a community hall, rather than in a Church, with no mention being made about God. It was pretty clear that there were no last-minute conversions.
When you go to RCIA (the program for people who are becoming Catholic) what they do is they walk you through everything that you need to know for your First Confession. Normally, people who are newly entering the Church will have completed the second stage of their RCIA process before going to First Confession. It’s not something that would be required or expected right off the bat.
At the end of the second stage, those who are going through the process of RCIA are present to the Bishop, and he authorizes their priests to be allowed to start giving them the Sacraments, including First Confession for those who have already been baptized before in a different church.
They would then receive their Sacraments of Initiation (Baptism, if they had never been baptized, Confirmation, and First Holy Communion) at the end of the third stage - normally, at the Easter Vigil or at some point during the Easter season.
First, one of the features of mortal sin is that it is memorable, so it’s unlikely that you would forget.

But if you did forget something, it would still be forgiven. If you remember it later, you would just say, “A sin from my past that I forgot to confess is …” at a later Confession.
You would name the specific sin, but you wouldn’t have to describe in detail every single incident - you would just say how many times you did it, or how frequently you did it, and then give an example of one time that you did it. For example, if you were a terrible gossip when you were a teenager, you can just say, “I gossiped every day from the time I hit puberty until I was half way through my last year of high school. One time I remember that I told my friends that another friend of ours had done something that really she didn’t do, and she phoned me later that day and told me that she didn’t want to be my friend any more. That was my wake-up call, and I decided at that time that I needed to break this habit. It still took several months before I finally broke the habit, though.”
They want to receive the Last Rites from him, which is their last Confession, the Anointing of the Sick, and their last Holy Communion. This reassures them of God’s love for them, purifies their soul of all sin, and gives them spiritual strength for their final journey to God.