Knitting this top down and seamless.

Cast on for back, sleeves and 1 st for each front (the cast on edge will be picked up when attaching the collar)
Knit first couple of rows then break yarn (ws).

Collar: (see note)
Cast on st for the right collar extention. Use a crochet hook to attach it to the centre back cast on edge to knit it as you go. Make sure you end on the ws.
Slip the sts on the needles next to the main body, break yarn.

Pick up the sts for the left collar extention with a small crochet hook. Be precise to pick up exactly in the loops of the cast on and the join will be joggless with a very neat ridge underneath. The last neckline st will roll slightly and give a lovely finish.

Just like previously, attach the collar to the body as you go, make sure you end with a ws row. Slip them on the main needles.

Note: I had to fiddle for a while to find a way to attach the collar.
Another method would be to cast on an extra st for the right collar and work it separatly, keep sts on hold, break yarn.
Pick up st and work left collar, keep st on hold
Sew collar to the cast on edge of the body
Slip collar sts on to the main needles, carry on seamlessly.

Rest of the body:
Next rs row: knit left front collar, sleeve, back, sleeve, right front collar

The hardest thing working it top down is to work out the pattern placement and incorporating the raglan increases at the same time…

I had to add 6rows to the raglan to have a comfortable armhole.
4 body increases as i made it longer

Sleeves: too tight, so i picked up 2 extra st. I tried it on to work out where to put the following decr. (3dec, then a 4th dec on the purl row before starting the ribbing)

All the ribbing done in 5,5mm.

fit around the armhole is okay but I think it would be much better by casting on more sts for the sleeve (and move the front raglan forward)

20g left

Update 2019: frogged!
I will definitely knit this cardi again with mods to impove fit.