I expect that either you’ve Avoided the whole time, which is whatever, or you’ve switched to a different weapon and therefore lose the Advantage (If you’re Blocking, Striking, Beating, etc., you’re not doing it with your spell so you switch.) I don’t know how or why you wouldn’t cast the spell. Just trying to abuse the spell’s length for someother actions? Sounds sketchy.
“Spells must be completed uninterupted,” page 511. If they aren’t, you have to make a Tax test and incur the Failed Casting results – Die of Fate for Unwanted Summoning, etc. My read is that if you change spells, you’re functionally changing weapons too. In fact, I reckon you might not want to do too much else in an exchange between starting the casting and finishing it.
The action says that you must do all the actions consecutively, but you still can position with a spell and then do block/block/cast spell for instance, no? Actually, I guess not, since you roll your weapon skill to block and that means you need a weapon.