This is my current assumption, based on the "Fireball" statement. Let's say I started with 1 level of Cleric and then took 4 levels of Sorceror. Upon hitting overall character level 5 and selecting Sorceror, my caster spell slots would be the equivalent of 5 total levels of a full caster, meaning I would have 4/3/2 spells slots and thus I could select Fireball at level up. Is that correct?
As far as I remember, this is how 5e works RAW. You don't have spellslots by class, you have spellslots and various classes sum up to create your "spellcaster" slots. Every full caster count as 1, paladin and ranger counts a 0.25 and eldrtch knight and arcane trickster counts as 0.5. Again, I'm going by memory so I can be totally wrong. Anyways, by RAW, if you are Cleric 4 / Sorcerer 1, you can cast your 1st level sorcerer spells with a 3rd level slot, or your 2nd level cleric spells with 3rd level slots. You can't learn 3rd level spells with neither of said classes though, so as a cleric 4 /sorcerer 1 character you would only know cleric's spells up to 2nd level and sorcerer's 1st level spells.
Since this is the base already of the system everything the dev said should come on top of this, otherwise it would be pointless to specify it.
About the answers for your questions, I think nobody knows at the moment.
