The main thing to understand about bustles is that they were simply part of women's clothing. During the periods of time that they were fashionable, about 1870-1876 and 1883-1889, nearly all Western women except the poorest and most rural wore them. From the late 1840s, some form of skirt support was typically worn: at first petticoats made of crinoline (a mixture of linen and horsehair), then hoop skirts; by 1869, the hoop had become fairly narrow and conical, and women began putting a small structure of padding, hoops, or crinoline on top of it to create a bustle shape. As the silhouette was normalized, bustles were made by changing the shape of the hoop skirt itself, or with a longer structure that hung down in the back without a full skirt component, rather than by doubling up. So most women of the bustle period(s) were very used to managing supported skirts of some kind - they weren't suddenly having to deal with an unfamiliar protrusion or circumference the way that reenactors do. Think of it like driving a car: if you're a frequent driver, you're aware that your car is a machine propelled at terrifying speeds by explosions and can easily kill and maim other people and yourself, but you're not consciously thinking about it all of the time when you're driving. It's just an extension of yourself.

Some examples:

A version made as a nearly full hoop skirt with the top of the back made to stick out a bit more

A similar one with a different construction

A bustle that only hangs behind the legs, no skirt

A small wire bustle that only sits behind the butt (Sorry for the Pinterest link, the museum is now defunct.)

Because bustles (and other skirt supports) were a completely normal part of dress, everyday life was very possible in them, including sitting. They were, in fact, often made to be accommodating to sitting! The entire structure was collapsible, which is why the above are all displayed on mannequins - if they were taken off them, they would simply fold down into a flat circle or semicircle. There is no vertical stiffening. (Well, the fourth one has vertical stiffening in that the hoops rotate around a point, but they all fold up on that hinge.) If the wearer sits, they collapse.

They were not difficult to put on, and were all - as far as I'm aware - made to be put on by the wearer. As you can see, all of the above examples fasten in front. The wearer would either step into it (if a full skirt) and bring it up around their legs, or take the waistband and wrap it around their waist. If a woman did have assistance, whether from a servant or family member, that might make things easier, but it was already so easy that we're talking "someone else puts on your socks for you" levels of leisure. Taking them off would be the same.

They did not really complicate using the bathroom, because by and large people weren't using flush toilets. Women's undergarments were typically bifurcated and made with an opening along the crotch; they could easily step over and crouch down on a chamber pot without having their bustles in the way or having to fumble beneath them. You absolutely would not remove your bustle to use the chamber pot, because that would mean entirely disrobing.