Toledo is a great winger zoo but there are a few animals you may miss including, many African species (possible to see if you do the train), and a few tortoises (galapagos and leopard). The phesantry and flamingo habitats could also be closed. Almost all of their larger animals have indoor viewing but if you do take the train you would be able to see the African species off display. As for must see exhibits I would say are the museum, aquarium and aviary.
Did the reptile house ever close for rennovations? I went in May and it was very much open and with nothing noticeably different.
I don’t believe so, they are working on the back currently.
Some fruit dove species (black-naped and pink-necked, especially) are actually fairly common in US zoos, so there will be plenty more places you can see them. That said, Toledo does have a nice fruit dove collection- I saw four fruit dove species during my October visit.
I have no idea about winter, since I visited in October, but I can answer some other questions. I found the indoor area available to the elephants particularly impressive, although I don't remember if an on-show indoor area existed for the rhinos. For exhibits, you shouldn't have much problem seeing everything in one day- I saw the entire zoo (including some exhibits multiple times) over just half a day, though it all depends on how long you spend at each exhibit of course. ProMedica Museum of Natural History is a definite must-see, in particular the "Hall of Venom" on the top floor with an impressive snake collection and the poison dart frog walk-through that's on ground level. The Aviary is the other big must-see, and while the exhibits themselves aren't overly impressive the Reptile House shouldn't be missed either. While less prominent of an exhibit, I was also very impressed by "Flamingo Key", their walk-through aviary with flamingos, various waterfowl, etc. The Pheasantry is nice, and fairly unique, although I'd argue the aquarium is just okay.
There is an indoor viewing for the rhinos near the elephants.
I would not recommend trying to see the whole zoo in just half a day, although as @Neil chace has said, it is possible. Last summer there were a few days in the week that the zoo was open until eight o'clock. I remember the website stating different open hours than was the case, so I recommend you call for accurate information. Some areas of the zoo close early, such as the Venomous Snakes room (Venom Hall is always accessible, just not Venomous Snakes), so definitely prioritize seeing the Museum of Natural History first.
Just to add to this:
I completely disagree. Four hours is not enough time to see this entire zoo properly and to take time to look for all of the rarities. I was there for 6 hours (from open to close) when I visited and I felt very rushed.