Legendary Dude is right, in that the nature of Curse of Strahd doesn't lend itself well to prospectively breaking it into "episodes" or "adventures." It's designed for the players to be able to meander along their own path, following such leads as they like.

But we've got lots of data from other AL materials to inform a scheme. Specifically, I tabulated the downtime and renown rewards from the twenty-three (mostly seasons 2 & 3) Expeditions modules I had on hand. These included five 1-hour adventures, eight 2-hour adventures, and ten 4-hour adventures.

All of the 1- & 2-hour adventures rewarded 5 downtime days. One 4-hour adventure also rewarded 5 downtime days, while the other nine rewarded 10 downtime days.

Most (18 of 23) modules reward 1 renown point to any faction member participating.^* 13 of those 18 also conditionally reward another renown point to members of some specified factions. On average, it works out just over 1 2/5 renown points available per 4-hour adventure.

Summing this all up, AL practices would indicate the following scheme:

Award 10 downtime days per 4-hour session. Award one renown point to all faction members per 4-hour session. (Story-)conditionally award another renown point to members of 2 factions, on average, per 4-hour session. (And spread the love among the factions.)

Personally, I think that's too complicated. I've gone over to "rewarding" downtime as real time. That is, for every day that passes in real life my players earn a downtime day. (That way they've got more to play with when we have to miss a session!) I also think that's way too much renown--remember that the Expeditions format contemplated casual less-than-every-week play, the likelihood that a player would be using different characters week to week, &c. I would recommend dropping the "flat" 1 renown per session, and just stick to 2 or 3 conditional ones, as it makes sense in the story.

So, my best recommendation, after a few years of running AL games:

Award downtime as above, or as real time. Award renown points to two or three factions-worth of players, conditioned on meeting story "goals" keyed to each faction, per session.

^{* - the five that don't are the five linked 1-hour adventures. Each of these are aligned to a different faction and reward members of that faction 1 renown point at its completion. In other words, if we consider these as one 5-hour adventure, it's another one which gives all faction members 1 renown point.}

I'm going to attempt putting together some RAW information that I am able to find.

After reading through the relevant sections (Town of Vallaki) there is no mention of the reason for the characters appearance. Only his duties around the town (such as bullying people, and being a henchman). There is also some mention of his creepy doll collection in a few spots.

As far as the origin of his deformity - here is some information on how it started.

"One morning their father and their uncle took them fishing on Lake Zarovich. On the way back to town, a dire wolf attacked Izek and bit off his right arm." - Page 231.

It continues to mention as well that...

"Unlike his sister, Izek was born without a soul." - Page 231.

Lastly, here is how his arm was returned to him (in regards to what is written).

Fiendish Gift. After years of doing Baron Vallakovich's dirty work, Izek awakened from a drunken stupor one morning to find that he had grown a new arm to replace the one he had lost. The new appendage has barbed spines, elongated fingers, and long nails. He can create fire with the snap of his fiendish fingers and has used the flames to put the fear of the devil in every Vallakian. - Page 232

There is no actual mention as to how this happened. However, the name of the attribute leads me to believe that...

It is likely a gift from Strahd/evil beings who support Strahd.

However this is only a guess currently. It has been pointed out in the comments that additional fluff information exists in AD&D2E regarding evil acts committed changing your character in fiendish ways. This looks to be what is happening, and almost certainly is a gift from the Dark Power.