#Ruby, 405 403 401 bytes
A wise man (Jamie Zawinski) once said, "Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems."
I don't think I fully appreciated that quote until I tried to solve this problem with recursive regex. Initially, the regex cases seemed simple, until I had to deal with the edge cases involving letters adjacent to brackets, and then I knew that I was in hell.
Anyways, run it online here with test cases
->s{s.gsub!(/{(-?\w+)..(-?\w+)(..(\d+))?}/){x,y=$1,$2;a,b,c=[x,y,$4].map &:to_i
$1[/\d/]?0:(a,b=x,y)
k=a<b ?[*a..b]:[*b..a].reverse
?{+0.step(k.size-1,c>0?c:1).map{|i|k[i]}*?,+?}}
r=1
t=->x{x[0].gsub(/^{(.*)}$/){$1}.scan(/(({(\g<1>|,)*}|[^,{}]|(?<=,|^)(?=,|$))+)/).map{|i|i=i[0];i[?{]?r[i]:i}.flatten}
r=->x{i=x.scan(/({(\g<1>)*}|[^{} ]+)/).map(&t)
i.shift.product(*i).map &:join}
s.split.map(&r)*' '}