I don't know this for sure, however I don't think separation would have anything to do with you losing your benefits eligibility. You are still married and still on his family coverage.
It sounds like you have your doubts about his faithfulness - his concerns about his appearance, your checking his phone for "evidence", etc.
If nothing else, isn't it worth getting counselling? Don't you want to be able to tell your kids "we did everything we could"?
You say you're not in love with him, yet you're 7 mos pregnant too. Obviously sometime within the last 8 mos you've been intimate with him. Don't you think there's a possibility that you could fall BACK in love with him if some things changed?
I'm not about to blame your pregnancy/hormones for any of this. But just know that lots of women get annoyed with their husbands, feel fat and unattractive, etc. when pregnant. Heck - I sure did!
Please, try counselling. Wait until WELL AFTER the baby arrives to make any permanent decisions. That way you can talk to a professional, try to work on the things that have been annoying each of you and give it a good faith effort.
At least then you can make an informed decision and say it just wasn't meant to be. In the meantime, you can get some counselling on your own, save some money now and develop a plan for how you would handle things IF it comes to divorce.
Best wishes Mama. It sounds like you have a lot on your plate.