I haven't made any decisions, so I can't own them.
Deciding not to decide is also a decision in itself.
I do think you have not helped yourself by agreeing to him coming on this holiday. If you were here alone, you would have been spending the evenings with your friends, perhaps laughing or crying together about the situation, getting their support, or just taking a well needed break to get your mind off the subject and re-establish friendship ties. You would have had some distance from him and perhaps a chance to see things more clearly. All that is happening is that you are getting mired in a fog of indecision, which enables him to cut you out of the friendship group and get this couple on his side.
Perhaps I have misunderstood and they are more his friends than yours, as the woman is his colleague? (I didn't see your previous thread.) If they are equal friends of yours, then I think you should lay your boundaries down tomorrow and tell him "You said you would make yourself scarce in the evenings. That hasn't happened. Why?" and then tell him you'd like to spend tomorrow evening with friends so can he please fuck the fuck off make himself scarce. you know, like you've done every fucking night so far.
There is obviously some things you've done that you now wish you'd done differently. Don't feel that you're somehow obliged to keep carrying on doing something or following a course just because you've started on it.
Deciding to stay together is perfectly valid if you feel you can put this behind you. However, I'd caution that he will almost certainly have more affairs, and with him working away so much, you will probably never find out. If you cannot live with that, you are best off leaving. If you feel you can make your peace with "he's probably at it but as long as it doesn't affect me and the kids and he doesn't rub it in my face..." then stay. Thousands of women before you have done it and thousands more will in the future. Monogamous sex life isn't the be all and end all of a marriage.
That said - again I haven't read your previous thread - if there were suggestions that he was going to leave you for OW then you MUST protect yourself and the DC. Get employed, even if you have to take a hit on overall household income initially to pay for childcare. If he pulls this shit again 5 years down the line you need to feel confident you can trim sails and survive on your own as a single parent.