Impregnating Forsaken hasn't been implemented yet other than the initial impregnation you can do to them when they are chosen. However! Luckily CSDev said the update for this month will allow us to give defiler abilities to our forsaken finally.
The training uses the punisher upgrades similar to how defiler and suppressor is used for some of the training methods, so we'll be using our impregnation commanders on the forsaken and effectively "teaching" them to inseminate as well.
(with them giving additional bonuses such as using drain commanders on forsaken giving us additional EE for each obedience gain during that training session in campaign mode.)
And in regards to use of forsaken, I felt similarly about their effectiveness until the campaign mode was added. As I went through loops with shorter and shorter deadlines, the ability to use a forsaken that can capture a chosen for 4+ turns and dealing decent damage in the early game becomes almost integral rather waiting to mid-game-ish to get a commander that lasts that many turns and the upgrades along the way. And once you get bonus damge % from them being tempted, aversion, or superior forsaken, they start to outdo commanders even without heavily training them. Similarly sometimes you need to train them not to raise their expertise, but just to raise their motivation and "keep them in line" as it were.
And all this sort of influences how plan out my loops now, whether I aim for an Imago to end it early and use that time for training my forsaken, trying to intentionally make chosen friendly with each other to use it as leverage when training them, or even not choosing to break all vulnerabilities on a chosen purposely or only in certain ways to ensure they have decent values when they are forsaken. This is just my experience and thoughts on them though.
And to answer your other question I believe CSDev made a post on here recently saying that we might be getting a sex system with our forsaken as early as the end of March, at least that's how I understood it, I'll quote the post directly under here