1. She always gives you the prayer, either when she dies or before the last fight.A protip: supposedly if you dont use Viridiana help in boss fights she gives you a prayer that is op and let you breeze the last figth, i never used it because i believed she was a requirement for the true ending and got the prayer but someone pointed me that it was not necessary and you have to actually see her die to get a achievement
2. It's not really that useful. The best defense is a good offense, the faster you blast the guy's face off, the fewer projectiles he can shoot at you, so IMO all the mana should go into Debla of the Lights if you're having problems (I did him in with no cans or prayers, skill gauge: ragequit Undertale in the crystal flower maze). Fireballs is what people seem to be having most trouble with; walk them to a corner, stick there for a bit, then roll away.