Snow, I'm sorry you have found yourself here, but SI is a wonderful community. There are many, many people on this site who at one point were sitting where you are now, feeling scared and overwhelmed, and they have built peaceful and healthy lives on the other side of infidelity, separation, and divorce.
On the in-laws:
Assuming you separate, you mention your husband said he'd go back to his country. Are your in-laws in another country as well? Or living down the street? And your kids are at least 8 or 10 years old (if they were young children when you H started cheating on you several years ago)? If you're not relying on your in-laws for childcare, then the answer is straightforward: you separate from them, too. If you want them to have a relationship with your kids, then you support that within reason while minimizing your own contact with them. Support your kids, don't bad-mouth their grandparents, but your interaction with the in-laws to the transactional details of arranging visits. If the kids are old enough to decide what kind of relationship they want to have with their grandparents, then you can factor that in. If the in-laws are much more involved in the kids lives, that's a little trickier. In that case you'd want to think about what reasonable boundaries are for their interactions with you and your kids - then set those boundaries and stick to them.
On protecting the kids:
Your instinct is natural. Two thoughts to offer here. One is to figure out what you want to protect them from and focus on that. If you think your H is going to be angry, toxic, and harmful, then it's about finding a way to co-parent that manages this dynamic (and if he has serious issues, that would likely come up in custody and visitation negotiations anyway). If you're trying to protect them from a toxic relationship between their parents, the good news is that you have a lot of ability to control that. No matter how he acts, if you're clear, reasonable, firm, and calm, the kids will see that. If you don't speak badly about your husband in front of the kids (save it for your therapist or best friend!), that matters. What you can create for your kids after D may be far better and healthier for them than what they're seeing of your H's behavior during marriage.
The other thought is that while having two supportive, loving parents who can model ideal behaviors is what we all wish for, there are plenty of kids who have healthy and terrific lives even if they had only one parent showing those behaviors. You have been and will continue to be a huge positive influence in their lives, so don't think that D automatically means your kids' futures or ability to have healthy relationships of their own is doomed.
You'll get great advice here from people who have been through it. Hang in there!