First, try to put the ends of both dumbbells onto your thighs, just above your knee, then when you are ready, let your legs give you a slight kick to lift up the dumbbells.

No,hold the grips normally and hold it vertically on your thighs,then kick up.

For decline dumbbell bench press you'll need to get someone to hand it to you.

Dude, I get what you mean but you're gonna fall off the bench if you approach and attempt to lie on it from the side. It's okay to do so when getting onto or off your bed but not a bench. I used to have spinal problems too. But to attempt to lie on a bench in that manner would most likely result in a disaster.