Welcome to the forum Blaine, I will answer as best I can both as a cattleman and as an ex meat inspector.
The main objections to slaughtering pregnant animals are ethical, though sometimes late pregnancy can lead to a carcase being condemned due to poor bleeding out (fevered.)
The later into pregnancy the heifer, the more feed you are putting into calf development rather than slaughter weight, so you wouldn't want to go beyond 5 months. Heifers can concieve as early as 6 months, but usually 8 months is the age usually associated with first heats, pregnancy at this age is undesireable for many reasons; difficult calvings and stunted growth being the most obvious. Slaughtering older, first calvers, will still produce a good carcase, but will effect grading if sold to an abattoir, for private sales to a niche market, they will be just fine but they should be 'hung' for a minimum of two weeks to enhance flavour and tenderness.
If you do not want to seperate your cows and calves, why not consider AI and forgo all the heifer hassles? You need to seperate the calves at weaning to let the mothers dry up their milk production, or else some cows will let their calves suckle untill the next one is born!
hope this helps with some of your questions.