Hi Spook.
In truth, I do not see your marriage as irretrieveable in the long run. There is lots about the circumstances, (the financial pressure, his age, - have I got this right, early 40's? -, the not-as-bright-and-articulate-as-you-ness of ebb) that spell mid-lfe crisis or other aberration. But from that he could either move back to you, or away.
But I will be honest and say that my heart is in my mouth for you over your hopes for Ibiza. In the first place, he hasn't suggested it. In the 2nd, if YOU do, he may well accept as a way to spend time with the boys, and kid himself that he's being good to you. But how would you know he wasn't contacting her? You would be jumpy every time he went to the toilet, suggested he have the boys for an afternoon to play football with them - and text madly. At some stage you would check his phone, his bags. He would resent your suspicion, and blame you, not himself and feel as if he had had a tense week. AND after your week of make-believe happy families, how would you feel if you found he went straight from the airport to dinner with her?
If he suggests Ibiza together, I think you should only do it if he is committed to NEVER contacting her in a personal capacity again, and if he is serious about wanting to try to make a go of it with you.
I'm sorry if this sounds negative, or not understanding of your position. But I think that a week that went wrong could do more harm to the long term prospects, if they do exist, and would be very very painful for you. But I don't think you are wrong to give up on hope, or be prepared to put energy into re-building things if the time, and his motivation, are right.
Are you afraid that you are deepening the rift between you and 'burning your bridges' by keeping some distance? You have told him you are lonely. He must know that you still love him.
I have never known a relationship be repaired until the straying party pro-actively ceases contact with their 'affair'. I hope, every day when I log on, that he will have come to his senses, ditched her and is refusing all her texts and messages- you'll know when he does, cos you'll get a bitchy call - (god, my Dad's gf's mother rang ME to rant about how cruel he had been to ditch her)- and is begging you, BEGGING you to come back. But I fear that if it is going to happen, it will take longer than this.