'05 - No start no jump, orange blinking light on start button

Hi all, Looking for inspiration on a 2005 Prius. It belongs to a friend. The tailgate was left open for a while, drained the battery, and now there's a no-start problem. When I first played with it, there were no sounds, and no dash lights except the blinking orange light on the start button as I tried to start it. About 12.5V on both small battery and the jump-start lead in the fuse box (to chassis). No headlights / dome lights / anything except the blinking start light.
I went through the fuse box, and from someone else's post here, started with the dome light fuse - it was broken. Swapped with a spare, and there's a little progress but still no start. Headlights, dome lights, door lights dash lights seem to work now. But still blinking orange light on the start light, and no start. The only dash lights that come on are the door-open light, the immobilizer light, and the turn signal lights. When I try to start it, the check engine light comes on for a half second then disappears. A couple other weird symptoms:
Brake lights don't come on, but pumping brakes hard seems to cause a weird sound - maybe the charger?
The immoblizer doesn't turn off unless putting key in the key ignition.
The lock / unlock on key fob doesn't lock the doors, but the green turn arrows on dash light up a second when trying the fob buttons.
Just replaced the small battery, and the key fob battery. Still 12.5V on both batteries. And trying to jump start does nothing to change the situation. Charging it overnight with battery charger / leaving it attached to running car for 15 minutes = no change either.
I went back to the fuses, and checked them all - on the little battery, in the big fuse box and the little one. Found a broken little fuse - EMI or something. Swapped with same size hazard lights fuse (same amperage) and tried car: the green light came on the start button a few times but still couldn't start the car. After trying 3 times, it went back to blinking orange.
There is a couple-second buzz that comes repeatedly from the engine compartment as I try all this starting. But that's about the only sound. Haven't seen any other dash lights / the battery system monitor light up at all.
Any suggestions? (and thanks)

 

Thanks bisco. So I searched harder on this forum, for a couple hours, and found the thread about the fusible link with the pic of the 100 amp fuse being broken. I'd never worked on a Prius before, didn't recognize that big white box as a fuse. So when I said "checked all the fuses", I hadn't checked all the fuses. That 100 amp fuse was broken just like in the posted pic in the other thread, and jumping with pennies (thanks TMR-JWAP) confirmed the prob by getting the car to start up. The owners probably jumped with reverse polarity. Sorry for bothering everyone. Thanks for the help.

 

BTW - pretty hard to find that fuse block online. I'll try the boneyard tomorrow. Do you think soldering it back together would work? It's supposed to be a safety fuse, right? (and it's not my vehicle).

 

Leadfoot's pic just made my day.
Thanks M in KC. I'll stop at the Toyota dealer if I don't find the fuse intact at the boneyard. And thanks to everyone else in this forum.