I wouldn't put the sleeping bag on top of the duvet, because I was worried it was dangerous (duvets reflect body heat so risk of overheating, duvets are also much softer and looser than a sheet stretched over a mattress so could be a suffocation risk) and also I found it easier to have DS cuddled up right next to me, he used to fall asleep on the breast. I didn't feel comfortable rolling away from him when he was tiny because the closer I was to him, the more aware I was of him, even in my sleep.
To keep warm I used to pull the duvet over my legs and up against my back. I would also wear long sleeved pyjamas and on colder nights a dressing gown, hooded zip up top or fleece, etc. I dressed DS in a sleepsuit and vest and put a folded cotton cellular blanket over him, I also pulled this over to cover my tummy which was quite often exposed. He was born in October and this was fine all through the winter, so should definitely be fine for Spring/Summer!
Once DS got a bit older and I was starting to get fed up of the back/hip pain from sleeping in the same position all night (I usually roll all over) I started to scoot him away from me when he had fed, into his little space in the bedside cot. Around this time I also started using the baby sleeping bags, because I wasn't close enough to him or aware enough to him to adjust his blankets all the time as the temperature dropped in the night or as he kicked them off etc. Having him in the bedside cot was great as I could cuddle DP, sleep on my front, sleep facing away from him, anything, but he was still close enough to feed without too much disturbance.
Also my biggest tip for co-sleeping: Buy a waterproof mattress protector (a decent one not a horrible crinkly sweaty one) - has saved our mattress from blood, sick, milk, wee, poo and knocked over drinks/bowl of water (nappy changing). Much cheaper than a new mattress and when DS is ill it's nice to have one less thing to worry about.