I think these are excellent questions. I will admit I do not always understand these things, either, but since I have no question about church authority, I feel content to believe while I continue to learn.
There is a difference between material sufficiency and formal sufficiency. Scripture is materially sufficient, in that all we believe is contained or at least implied, in holy scripture, and we can, through it, come to the knowledge of all truth. Formal sufficiency, however, says that it is all contained AND we have no need of any additional interpreter or authority to help us in understanding it, and that is not the Catholic position.
The problem with Sola Scriptura is that it totally removes scripture from church tradition. It doesn’t look at how the fathers read it, it doesn’t look at how the early Christians or the jews of the OT looked at it. It takes scripture and lets everyone interpret it based on their modern day understanding, which very, very often leads someone to error. And at the very least, it removes a great deal of wisdom that is to be gleaned from the scriptures.
As to why we need those “extras”, because we have the Apostolic Deposit of Faith. Both oral teachings learned from Jesus and passed down orally, AND the written texts, are the Word of God. We do not dispose of those oral teachings just because they were not written down.
Veneration of the saints, you ask, do we need it for our salvation? One one hand no, and on another yes. We do not, as individual Catholics, need to venerate the saints, pray to them, have their images in our homes, or pray novenas in order to be saved. However, church tradition says that their prayers are incredibly powerful, and that without them, many may not be saved. Even if we do not invoke their prayers, they are praying for us, individually and for the world.