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Ours didn't come with a sling, but it had a detachable plastic part on the bottom to keep baby somewhat steady... We were gifted a sling and didn't use it once. Either way, I feel like you still have to hold the baby somewhat since they have zero head/body control....
As for swaddles, I had what seemed like every brand of swaddle out there (desperate times call for many late-night Amazon orders!)- but realistically, I would say at least 2 swaddles, preferably 3, in case of accidents or what not.

We used a hand me down tub for LO1. I didn't even know about the sling until I had to buy a tub for LO2. Obviously I didn't miss it.

I found the sling to be annoying past a month. I think you could lay a towel in the tub to presently slippery baby from sliding down

As for swaddles I think it depends on what you plan to use them for. Do you mean a swaddle blanket like Aden and anais? Do you plan to actually swaddle the baby with that for sleep and naps? We have probably 8 swaddles (I like swaddle designs). We use them for evvvvverything except swaddling the baby for sleep. Stroller blanket, change pad, burp cloth, nursing cover, furniture cover for spit up. We used a halo or summer infant swaddle for sleep. To that effect I was super thankful I always had a clean swaddle on hand. Actually thinking about it I think I have 11 or maybe 12. Whoops

I think maybe you can try using the tub first, and then get the sling if you feel more comfortable with that? I feel like everyone bathes their babies so differently!

For swaddles, I got soooo many different kinds but I honestly really only used like 2-3 in the beginning and then I moved onto merlin sleep suit.