There must be something in the middle of that line to hold in the double bonus relay. You may notice that when you press the double bonus relay in it will immediately release when you let go as it is not a latching relay that can lock on itself, and it is designed to obviously stay on and stay pulled in until the bonus is drained and up until that point while you play the game to activate the feature and the corresponding lights. Are there target relays for each bumper that also lock in? They could be what completes the constant 30v path to lock on this relay. Something else needs to lock on to make this stay pulled in because if it was only pulsed it would light up for a second and then go off again because it's not designed to hold itself in like some other relays are. Something else must lock on to hold this in too, I'm guessing each of those targets/bumpers has a relay too that locks on which is also how it would preserve which bumpers were hit to light that bonus while it waits, kind of like "memory" except they just lock until you drain, either with or without the sequence finished, with or without reward, and each one has a switch that are wired together to complete power to the double bonus relay I would assume. Each of those switches need to be clean, free of grime, and gapped correctly for this to work.

You can test the coil by hooking one end to a jumper wire with an alligator clip and touching the other end to the solenoid power output on the transformer, or I've also done it from the 30v hold relay. Alternatively, you could do it where the fuses are located too and this would probably be easiest. I think your ground would already be set up for you so you don't need to worry about it.