Hi Sadie, lovely to meet you. Introductions is a very, very difficult time, and you have my sympathy. My little girl was 10 months when we met her, and my older dd was 4 (and started school just four weeks later - NOT ideal). From what you have written, I strongly doubt your ds2 will take ages to attach to you. He is strongly attached to his fc, and when that is removed he will be bereft. Yes, that is awful, and the guilt is excruciating, but he will NEED to replace that attachment figure as soon as possible - and you will be there. It is far more risky to take a child who has learned to not trust or rely on any of their caregivers.
My dd was very, very strongly attached to her fc - they adored each other - and I can only guess at how distressed she was to lose her (early photos show a very sad little face 
). But she clung to me like a little monkey; I wore her in a sling or carried her around for months, and she transferred her attachment quickly and easily.
Having an older sibling is a very mixed blessing, I think. On the one hand, there is no way you can give both children all the attention they need in those early months. You need to give as much as you can to the new arrival, but you can't just abandon your firstborn. No easy answer to that. On the other hand, IME at 10 months old children just adore older children. My dd2 was just fascinated by dd1 from the start, and I think that really helped us bond as a family.
The early months aren't easy, but they are also very special. My advice? Keep calm, get lots of sleep, buy a sling so you can keep ds2 close to you while you talk and interact with ds1. And come on here a lot to talk to the rest of us 