The beetle in your photo is indeed the larder beetle, Dermestes lardarius (family: Dermestidae). These beetles feed all kinds of dead animal matter. While they historically are most commonly associated as a pantry pest feeding on foodstuffs made of animal matter, mostly cured meats, fish or cheese, their importance as a pantry pest has diminished through time with refrigeration and better packaging. They can still occasionally be found in the home feeding on other animal products, including dried dog food, furs, feathers, leather, and wool. However, one of the most common sources of home infestation by this species is on masses of dead insects that have overwintered and died in wall spaces and attics, but even accumulations of dead insects on window sills and in light fixtures can be fed upon. Cluster flies, boxelder bugs, Asian lady beetles, and most recently brown marmorated stink bugs are all insects that may enter into the wall spaces or attics of home in the fall to spend the winter. Most of them will leave the following spring, but often many of them die in these spaces. The larder beetles can find their way into these spaces and feed on these accumulations of dead insects and build up large populations. As they reach the adult stage, they will disperse and often find their way into the interior of homes. So, if you have ruled out pantry items, dried dog food, furs, feathers, wool, and similar items as possible sources of infestation, then I would suggest seeing if dead insect accumulations are a possibility, especially if you have had infestations of these overwintering insects in the past. If you keep finding the beetles on the same area of the floor, they might be coming from an overhead ceiling fixture, a light with dead insect accumulations, or crawling down through the fixture opening from an attic or ceiling space above. If you are finding them scattered through the house, it will be more difficult to determine where they are coming from. If you can access where any dead insects may have accumulated and clean them up (vacuum, sweeping), that should take care of the problem. But, if these accumulations of dead insects are in wall or ceiling spaces with no access, there may be little you can do unless you hire an exterminator to treat the inside of those spaces. If you are only seeing a few beetles, then it may be best to simply vacuum them up when you see them and not be overly concerned. If you are seeing lots of larder beetles, then obviously there is still a major source of food for them somewhere that needs to be eliminated or treated. And unfortunately, as long as the overwintering insects keep coming back, then so might the larder beetles. I have included a link below to a fact sheet on larder beetles, or you can search the web for larder beetles to find more information.

http://ento.psu.edu/extension/factsheets/larder-beetle

From what you described, my guess is that these beetles are indeed most likely coming from the attic space above the bedroom. There are several possibilities as to what they might be feeding on up there. Since it has supposedly been sealed to keep the bats out, there probably aren't any larger dead animals up there, and a larger dead animal would more likely be a food source for different species of dermestid beetles other than the larder beetle. I don't know if larder beetles would feed on bat guano, although since bats eat lots of insects, there could be substantial amounts of insect parts in the guano that could be a food source. However, if the guano was the food source for the beetles, I suspect you would have been seeing the larder beetles showing up long ago, and it sounds like your "infestation" has only been noticed more recently. I still suspect it is more likely that the larder beetles are feeding on accumulations of dead insects. Screening over vents and other openings to keep the bats out, wouldn't necessarily keep cluster flies, box elder bugs or Asian lady beetles from entering the attic each fall to try to find a safe place to overwinter. The attic space is probably not well insulated, so even in there, many of those other insects could die from cold or other factors, and that might vary from year to year. Once the weather warmed up in the spring, then the larder beetles might find their way in and begin feeding and breeding on the dead insects. We have had some unusually cold winters the last two winters, which might have killed more of the overwintering insects. That might be why you are only noticing this problem now and haven't seen it in the past. If the attic door opening is in the closet near where you are finding the beetles, then that is most likely where they are coming down into the house. Even if that access door is supposedly sealed, there are probably still cracks or small openings around it that the beetles could move through. The beetle larvae often wander away from, the food source to pupate in cracks and crevices, which they might be doing right around the access door. I am not sure you can completely solve this problem with out someone going up and inspecting the attic. I am not an expert on bats, but I think the toxicity of bat guano is due mainly to the ammonia levels. After 6-7 years, I think the guano would be so dried out that that might no longer be as significant a problem. A professional exterminator might be willing to go up into the attic to check things out by now. If so, although it may be expensive, cleaning out the attic of guano and dead insects would be the best solution to solving the problem. And if accumulations of dead overwintering insects is the food source, the problem could still persist unless you can screen over vents with finer screen or seal up all cracks and crevices to keep the overwintering insects out. If they continue to keep entering and dying, then there will always be a food source for these beetles. If all that seems too much to do, then I would at least seal around the access door opening as much as possible to keep the beetles from coming down from the attic. Once inside the house, adult beetles are most likely to head towards windows in an attempt to move back outdoors, so I am not sure spraying the closet area with pesticides is necessary, since they won't stay there long. Again, the only way to permanently get rid of these beetles is to eliminate the food source. If that is in the attic, then that is where the cleanup will likely need to be done for a more permanent solution. At this point, we have covered the most likely possibilities that I can come up with. It is possible there might be some other unexpected food source, but again, that that will likely take someone entering the attic to do an inspection.