Brain in a Jar
Oracle Text
, : Put a charge counter on Brain in a Jar, then you may cast an instant or sorcery card with converted mana cost equal to the number of charge counters on Brain in a Jar from your hand without paying its mana cost.
, , Remove X charge counters from Brain in a Jar: Scry X.
Card Rulings
4/8/2016 When resolving the first ability of Brain in a Jar, the newly-placed charge counter will be counted when determining what spells you may cast. No player may take any action between you placing the counter and choosing which spell to cast.
4/8/2016 If Brain in a Jar leaves the battlefield before its first ability resolves, use the number of counters on it at the moment it left to determine what spell you may cast. That number won’t change because you can’t put a new counter on Brain in a Jar.
4/8/2016 If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs, such as awaken costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Lightning Axe, you must pay those to cast the card.
4/8/2016 If the card has in its mana cost, you must choose 0 as the value of X.
There are several cards which grant the ability to cast single instants/sorceries without paying their mana cost from various zones, such as Chancellor of the Spires from graveyard, Galvanoth from the top of your library, and Living Lore from exile. All of these examples expressly state that the card may be cast "without paying its mana cost" and their rulings include ignoring time restrictions based on card type.
As for Jace and Snapcaster, neither of these abilities state "without paying its mana cost". Unless expressly stated, spells must be paid for with their mana cost, even when cast from zones other than your hand. Furthermore, the "spells cast for free" ability is usually a "may" effect which usually follows the triggered abilities/spells resolution. At that time, you decide either to cast the spell for free, or send the card to the appropriate zone. Jace and Snapcaster on the other hand provide you the option of casting that spell until the end of turn. As such, casting those cards will follow normal timing restrictions on top of paying for its mana cost.
TL DR- The spell you cast ignores timing restrictions and is cast without paying its mana cost as long as its CMC matches the charge counters on Brain in a Bottle. As the image has been updated in English, hopefully this matter can be out to rest.
When casting an instant or sorcery card this way, ignore timing restrictions based on the card's type. Other timing restrictions, such as “Cast [this card] only during combat,” must be followed.
This could be real fun with Coretapper.
Also, worded this way, it ignores timing restrictions. It has to, because otherwise it couldn't allow you to play sorcery spells at all, since the the spell would have to go on the stack while this ability is resolving.
So, basically aether vial for instants/sorceries, although you must pay to charge it also can combo with proliferate or things that add charge counters, so the combo potential is there. Also, unlike Aether Vial you can't choose not to add a counter, so you can't just leave it at say 3 to keep powering out 3 CC spells.
As for fuse, all this card does is check whether the CMC of the instant/sorcery is equal to the charge counters. From the rulings for fuse cards:
" If you cast a split card with fuse from your hand without paying its mana cost, you can choose to use its fuse ability and cast both halves without paying their mana costs."
and
" In every zone except the stack, split cards have two sets of characteristics and two converted mana costs. If anything needs information about a split card not on the stack, it will get two values."
So yes, you can play it fused.
I would like it so much more if the second ability did not tap it.
It makes you jump through hoops to maybe sometimes get this awesome whatever, but then you realize the awesome whatever isn't really worth the hoops.