Is there a reason you’ve had difficulty with litter training? How is her cage set up? Is the litter in the litter box and nowhere else?
As for bonding, it would be better to have a temporary cage. 2×2 is just too small and 2×4 is small for a pair. You could make a larger area for your bun (and potential friend) using NIC grid panels to construct a condo.
No bonding does not always work out. There are a few cases where the bunnies in question just aren’t compatible. Most bunnies can be bonded, but it isn’t always easy. That’s why we recommend adopting an adult bunny from a shelter so their personality is known. You can take your bun there to pick out her own friend. That sets you up with the best chance possible for getting a good match. I wouldn’t only look at males since you could potentially miss a perfect match that just so happens to be a girl.
I’m a firm believer that bunnies do better when they have a friend. Mine have both benefited tremendously from having someone who can speak their language. Wooly is somewhat timid and Leopold has made him more outgoing. Leopold is deaf, so Wooly has helped him understand what’s going on in their little world better. They rely on each other and do things for each other that I simply cannot. That being said, two bunnies means twice as much food, hay, veggies, treats, and litter. Oh and poop, definitely more poop. And vet bills, definitely more vet bills. To me, the cost isn’t that much, but it different people might see it differently depending on their circumstances.
There are a ton of different methods for bonding two bunnies. What methods are used will depend on everyone involved and how difficult the bond is. Mine were relatively easy to bond, but that isn’t always the case. I simply allowed mine to spend time together during bonding sessions and they took to each other pretty quick (after Leopold was finished humping Wooly every which way). They worked out their relationship and that was that. Other people will likely describe to you a completely different experience because every bun is different and every bonding situation is different. And that brings us to your last question, the amount of time it takes will vary widely. Sometimes its in as little as a week or two. Sometimes it is MONTHS and sometimes bonding sessions have to be put on hold to give everyone a breather before continuing.