In order:
Exactly that, yeah.
One-night stands like that happen rarely, and saiyans have a strong cultural inclination towards responsible contraceptive use. So it hasn't come up...yet. Biggest nightmare for the Lords -- it will eventually, just by the laws of statistics. But in theory they'd try and get the mother married in as fast as possible. That said, in that case the Masquerade rests on a knife's edge. They're not above making it clear to the mother that she can stay silent or be silenced if she's not willing to marry in peacefully. They would very much rather the peaceful route, though, because they're not monsters and don't enjoy the thought of imprisoning somebody for life, even in a gilded cage. At the very least, it sets a bad precedent.
"Force," no. This is basically the same question as above; they give the human parent a shot to marry in, with the proviso that as the human doesn't have a House, they take their new spouse's by default. If the human wants to continue going by their old last name, that's fine; it's just that their children will take the saiyan's name. The human that keeps their name will just be ever-so-slightly outcast in the way that men who take their wives' names or hyphenate are in western society, and being human leads to a bigger outcast factor than that already. Second step is flooding the relationship-space around the human. Last step -- the emergency step -- is directly approaching and bringing them in on the secret. They get asked to hold their peace, and if they don't, then they are threatened. As the absolute last resort, absolute intractability would lead to the human being imprisoned for life. After all, it's not like they could feasibly escape holding cells meant to imprison saiyans. The grandiose threats of violence are just that; threats. Of course, if the human involved is the father of the child, and the mother has no desire to marry him, she just vanishes one day, he never sees her again, and he never knows about his child(ren). All of this being hypothetical, naturally; it's not yet come up.
