This seems to be what they generally refer to as a "cuff to cuff" sweater? Yes?
Visually, the "rows" are not horizontal, but rather, vertical?
You are not using a designer's pattern? No?
If I were you, I would take a look on the internet...for free "cuff to cuff" patterns...and just see what advice the designer gives for all aspects of a "cuff to cuff" sweater. It should help guide you through the basic process of cuff to cuff knitting. There are actually books dedicated to this type of garment! I love the look!
Will your sweater be a pullover?
But to address your question about decreasing for sleeves:
I am not quite certain where you are in the stream of things, but I think you are decreasing down for the 2nd sleeve, having knit the first sleeve, and the body parts, right?
Anyhoo, sleeves are usually decreased (or increased) AT EACH END of the designated decrease (or increase) rows.
For decreasing, just tinker around with using either K2T or SSK at the beginning of the row, and use the opposite at the end of the row. So, if you like the SSK at the beginning of the row...then use the K2T at the end of the row. They are mirror images of each other. One leans left, the other leans right, I forget which is which...but Amy has video clips about them.
Even if the pattern doesn't say so, I usually execute my K2T and SSK decreases on stitches 2 & 3 at the beginning of the row, and at the end of the row, I do the decrease on the 3rd and 2nd stitches from the end. I leave the very end stitches alone (for untampered-with 'edge stitches'...it makes for easier seaming later).
Well, you are a brave knitter! You have the heart of a true designer! Good luck! Post photos of your finished sweater in the WHATCHA KNITTING section!
About the slit: I don't have any advice for that. Maybe another knitter will! Also, remember to look up cuff-to-cuff designs on the internet, or maybe the library...and see what the other designers are doing about neckline treatments on the cuff-to-cuffs.