Perhaps I'm missing something, but it seems really hard to gain the mastery ability in spells you know - if you've invented the spell yourself.
There are no books on the spell (no lab texts either, but that made inventing it harder).
So, to learn the mastery ability, the only option is to Practice.
That'll take a while with only 5 points per season. That makes Flawless magic very good.
Supplementary question, how high do mastery abilities get? level 3 would take six seasons... reduction in botch dice is very nice, is it worth it though.
The section on "Books on Spell Mastery" states that two spells are considered equivalent if "they have the same Arts, level, Range, Duration, Target, and effect; essentially, if the game rule versions of the two spells are identical." (see pg 87)
So books on the similar spell's mastery from say, Durenmar, would work, and it states you can pick a new special ability if you like.
Advancement states on pg164 that casting a spell repeatedly for a season is worth a source quality of 5.
soooo, you've some options... maybe commission a copy of a book on mastery from a good writer (who's probably has flawless magic, too) for a few pawns and practice.
Well, if it were not for tractatus, no ritual would ever be mastered. Imagine repeated casting and spending vis all the time
By the way, how are you able to master an "instant death" spell without becoming a people killing maniac?
This problem has not yet showed itself in our saga. But it will soon, I bet.
So I would allow a house rule to do lab work on the spell and grant 4XP in mastery ability for each season spent this way.
I don't think that you need to cast the spell in it's entirety to practice it. You could target squirrels with your corpus death spell, you could do dry runs without the vis.
I would allow a character to practice without vis or murder.
If you change an animal into a human it can be affected by both corpus and animal magics (but not mentem). Mystery Cults p.22 the text box marked "Ringing the Changes".
You apparently don't need human essential nature to be targeted by corpus spells, you just need a human body.