You sure do have a rough row to hoe. I apologize for the implied judgment behind the word spawning; but I was thinking "why create 5 kids if you're not willing to marry their mother." What's going to happen with those kids if you end up needing to divorce--and you're right that that could really happen? But you're taking the responsibility for now as the test of faith (and I'd say personality development via responsibility for little ones. I know what an ordeal that can be because I have sole responsibility for a 65 yr old wife and a 28 yr old daughter who are both disabled by severe and incurable illnesses and in chronic pain.
i would definitely have the same suspicions that you do about your father, since he lost his own wife to dementia a good while ago.
I was concerned about your partner's mother because an unaccepting/unembracing mother plus genetics for bipolar can develop Borderline Personality disorder, which is extremely difficult to improve. But the key emotional trigger-button for flipping-out is Shame = anything less than praise for her personality and actions--even "sensing criticism" where there is none. Strict religion naturally breeds quite a bit of moral judgment-which can work when applied to yourself, AND as careful training for your kids, but usually BACKFIRES when applied to a partner whose mother was Narcissistic (incapable of loving any child but the Perfect one for HER reflection) or or otherwise perfectionistic (Obsessive-Compulsive Personality could fit some teachers). You can look up the list of characteristics for these Personalities: Borderline, Narcissistic and Obsessive-Compulsive.
Even if your partner doesn't have enough BPD characteristics or her mother doesn't have enough NPD or OCD characteristics for them to fit the profiles, the best road to take for making your relationship last for the 20 or so years you'll need to give your kids their chance at productive & happy lives would be to add an ongoing "good mother" therapist relationship to her pharmaceutical mood management meds: a woman old enough to be her mother, with enough warmth in her heart to love the way your partner's mother could not and enough training and skills to handle your partner's difficult behavior.
I wonder if you're worried about your partner's declining interest in motherhood and perhaps also in you, because you're also worried that some day you're not going to feel enough love towards her to want to preserve the family you have with her, and you're not going to know how to get it back. I KNOW THAT LOVE IS RENEWABLE, but not as easily as an overdue library book. That's why I'm advising you to invest in a psychotherapeutically trained Good Mother (as "Grandmother") for your family: You had yours at a younger age when you needed her, but your partner never did. And you don't have yours anymore now.
I have lived a similar life in that way: My mother was good-enough until I was 23 when she died, while my father was a Narcissist (who couldn't love anyone less than perfect= anyone). But my wife's mother was a Narcissist, and her Good Father died when she was 11. Now I've had to learn to be the good mother for both my wife and daughter, and it's an uphill learning curve.
I assume it could be tight financially for you to furnish a capable therapist for the long term for your partner--but you'd be investing in her good-mothering (under professional supervision) as well as her emotional balance for becoming a good wife. You'd need to interview lots of psychologically trained people beginning with your partner's psychiatrist) to find the right person and then developa way to afford her ongoing devoted services (bipolar & borderline therapy, marital counseling and parenting consulting). She would become a valuable member of your family.
Now I haven't addressed your concern about your father well enough. So let's have at least one more exchange of messages--we have a week or more--or I can also offer you Skype or email counseling outside this website as a "premium service."