One needs to take account here of different meanings of the word home. (And I sense I may run into problems with Americans, since their use of prepositions can be quite different to ours.)
Consider that 'home' can mean:
a) my own home
b) someone else's home
c) a home in the sense of a nursing or retirement home.
In the case of a) I could only use 'at'. 'I am at home', and I wouldn't normally use the definite article. I could say 'I'm at my/our home', but never 'in home'.
With b) I would most likely say 'I'm at his/her/their home right now'. I could also say 'I'm in his/her.their home'. I would be very unlikely to say 'I'm at (or in) the home' unless I had earlier referred to being at their place of business or in their garden, or somewhere else with them. For emphasis I could say I'm at the home now', but it would be better to say 'I'm at his/her home'.
In the case of c) You can say I'm at the home, or I'm in the home. You would always have to include the definite article unless you refer to it as the home where someone stays. E.g. My elderly father is in a nursing home. I could say 'I'm at his home now' meaning the home where he stays. I could also say 'I'm at the home'.
In Britain we never, as Americans do, use 'home' entirely on its own, e.g. 'I'm home', 'she was home when I called', unless we meant 'I have/she has arrived home (from being elsewhere)'