In combat, I'd expect only 1 action or 1 round spells to be used. (Monster Summoning is one that takes multiple rounds, but could be shorter, but you're probably not going to launch into that after an Initiative roll, hoping to get a 30+!)
My take on the difference between those two is this:
1 Action: This just takes the one Action Die -- if the Wizard has two Action Dice (as at higher levels) they could decide to cast a spell like this twice, or two in one round. Or do something else and cast a spell.
1 Round: No matter how many Action Dice you have, if you're casting one of these, this is the only spell you're casting this round.
So there really isn't an easy way to "interrupt" the caster mid-casting. If Judges want to say the 1 Round spells don't kick-in until the end of the round, or until just before the caster's action on the following round, that doesn't necessarily mean the caster is "waving their hands" the whole time — they may do their mumbo-jumbo and then the magic coalesces over the remaining time. If you start a ball rolling down a hill, you don't have to run along with it the whole time making it happen, you start it off and watch it go, completely out of your control once begun.
As for changing your mind after you see the roll but before the spell "happens", well, rolling the spellcheck is committing to the casting. The player can see the roll — the character
can't.But your idea of treating the result of a 'cancelled' spell as an automatic natural 1 is cool. That could be useful if a spell is cast at a target, takes something longer than 'instant' to get there, and the caster decides they shouldn't have done that and 'calls the spell back' to themselves...

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Gnome Boy
• DCC playtester @ DDC 35 Feb '11. • Beta DL 2111, 7AM PT, 8 June 11.Playing RPGs since '77 • Quasi-occasional member of the Legion of 8th-Level Fighters.
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