Guys thanks for all the input. It's really helpful because I like to bounce ideas around.
I've just been working on this now.... i've got yet another option which i'm thinking about... i've edited the previous post... but to make it easier here's the list of options again:
OK so assuming I can't fix the bug described above (and it's highly unlikely unfortunately), i'd really welcome any thoughts on what to do about it... I can think of a few options....
1) Leave it - just live with the bug and write it up as a known issue in the readme
2) Scrap the multi turn casting times for strategic summons, possibly increasing mana cost or something else for strategic summons instead to balance it
3) Change it so that the summons happens immediately, but the caster is immobilized for X turns instead. So this would be fairly similar to how it is now, except that the unit would get summoned at the start of the spell cast. Movement would be set to zero, and hopefully it would also be possible to prevent any spells casting from that unit for X turns also.
4) Remove the casting time but give the strategic summons a cooldown time. So basically convert the casting time to a cooldown time. Possibly also increase mana cost for strategic summons for balance (as in option 2).
5) Make it so that when you cast a strategic summons, the caster is unable to cast *any* other strategic summons for X turns where X is the power level of the summons spell. So Shrills summons are power level 1, you can cast another summons after 1 turn. A Death Demon is power level 5 summons so you can't summon anything else for 5 turns. So it's like a cooldown but affects all summons for that caster. I've tested this one and it seems to work ok.... the only downside is that for period where the particular champion can't cast any summons... all their summons spells are removed from the spellbook (rather than grayed out). Not ideal but probably ok.
For now i've ruled out #1... I actually like it the way it was, but I really don't like that bug.
#3 was basically trying to replicate the casting time... however although the immobilization is possible, preventing casting of any spells is not so easy. It just gets messy.
So i'm looking at going for either 4 or 5 and doing a release with that change and a few balance changes. What do you think about option 5?