Hi, if you're having trouble getting Lilaya or Meraxis pregnant you should stock up on the "Vixen's Virility" item. During a sex scene you can open your inventory and make a partner swallow an item. As for your character, you essentially want to max out your own virility through perks. If you're a demon you can also change your cum output, storage, and refractory rate, to rather ridiculous levels.
As for your other question it really depends on the specific npc and their traits/fetishes. Certain named/story based npcs can also have more unique mechanics, to my knowledge, than the generic random npcs. However there is a perk in the lust path of the perk tree called "Object of desire". Which will make your partners last longer in sex scenes. Are there any specific npcs, or traits/fetishes, that are causing you problems? The more I know the more I can help.