They're definitely a lot more robust than they look. It does tend to be torn skin that's more of an issue than broken limbs. Like monkeyfacegrace says though, you get used to it and the vast majority of the time it looks a lot worse than it is!! Nipped ears have been very common here whenever we've a new puppy, taking a chunk out of the ear of the next whippet up is apparently a right of passage 
Seagulls would very likely get chased if the whippet thought they were in with a chance. Mine will chase crows (who I swear enjoy taunting the dogs), wood pigeons and on a couple of occasions they've almost brought down silly pheasants who try to hide before fleeing noisily when they get trodden on. They'd go after squirrels given the chance but we tend to only see them on lead which usually results in hysterics from at least one of them so not had to deal with them going after them. Although it's not legal to hunt squirrels with dogs but if it's a one off and you did your best to stop it (i.e.e you're not going to the park every day looking for squirrels to set your dog on) I highly doubt you'd get into bother. I've seen several posting on breed FB groups where people's whippets have got squirrels but never heard of anyone getting into trouble for it.
Two from the same litter isn't a good idea at all, it's far more than double the work of one puppy and can be very detrimental to the puppies themselves to be kept together unless you're able to keep them separate most of the time and do everything like walking/training/playing/etc. separately as well. Have a read through this article if you're still tempted!! I had two only a year apart and that was bad enough.
If you're getting puppies I'd leave an age gap of at least 18 months although 2 years plus is preferable, depending on the older dog. DWhippet1 wasn't remotely troublesome during adolescence and we added DWhippet2 when he was 18 months old without issue. DWhippet3 on the other hand has been much harder work and he's only just getting to the point now (at over 3 years old) where I'd even consider getting another puppy.
Obviously another option is to rehome a pair together (whippets are often owned in multiples and seem to come up in rescues as pairs fairly regularly), get a rescue then look at getting a puppy once they're well settled or do the reverse, get a puppy then when they're past the teenage stage look at getting another adult from a rescue.