Like I said in my previous post, I don't know why outdoors would be different. I know about WLD from previous experience, and from knowing a lot of people who grow citrus commercially and have been growing them for dozens of years. Leaf activity, eg photosynthesis, reduces the colder it gets. So if it is cold outside, roots won't function, leaves won't function...it is essentially dormant. Perhaps this is why WLD does not occur outdoors. As stated, I don't have the reasons, I just know from experience the roots do not function below 55.4F. Whether or not you choose to believe this is your choice. A link to any site would not be suitable, to prove for or against, but if you want to know more, do a google search for winter leaf drop with citrus and read peoples experiences. From there, whatever you choose to believe is entirely your prerogative. Best way to know for yourself is to experiment, Buy a citrus, sit it in a cold south facing room (below 12.5C but above 5C) and see what happens. I keep all of mine indoors from September onwards when it starts getting cold, and I have a lot of foliage growth and blooms. I have quite a bit of fruit on the go too. I have only had to deal with one scale problem, an isolated incident that was managed after one oil application.

check out the "growing under lights" forums. lots of people use artifical lighting (fluorescent shoplights, CFL's) to grow their tropicals indoors(citrus, orchids, plumerias, ets.) even down south i use CFL's during the winter when i move my tropicals inside the house.

That's why I love plants, all they need is a little water, food and a lot of sun. Don't create much of a fuss. I'm a bit of a misanthrope haha. Plants are good company.

The flowers may dropping so quickly due to the change of environment (either more or less light) and the shock of transplant. The color looks fine. I doubt you have rot already, but...that soil looks too water retentive for cacti. You want to mix it 50/50 with perlite, pumice or small lava rock. I would also gently secure it to a stake or something else tall because it's likely to be top-heavy...maybe that's why it's tilting. Just a general suggestion for when you buy a new cactus or succulent: don't change a lot of things all at once. Bring it home, set in a spot with good light (dappled shade or morning sun only, not full sun), and let it acclimate for a week. Then if you need to repot it, repot it and let it sit dry for a day before watering, and once again acclimate it to more sun very slowly. Start with dappled shade for a week, then add two hours of sun per week. A tall plant like that might be top-heavy as well so you might have to (gently) secure it to a stake.