Your response to this situation is perfectly normal. Things I would recommend:

Please at least mention it to your wife and mention that at minimum it is “affecting you emotionally.” It is not fair to her for you to return and be so worked up and not have an explanation.

Clients are understanding if they have an explanation. I would say something along the lines of “my coworker has a personal issue to attend to.” And a lot of times that is enough.

Though it is hard to deal with, remember that you are still here and made it past that time before. Though this sort of thought line is always a struggle, it is possible to push through no matter how difficult it may seem. What you said to your coworker is likely sufficient for now. Perhaps a reminder message saying something like “hope you are still doing ok” or “don’t push yourself to come back too hard, take time to work through things” would be appropriate on rare occasion until they return to the project.

These things are tough for everyone and nobody deals with it the same way. Since you don’t have access to your therapist, perhaps finding an outlet like journaling might help you work through your side of things even if you do so temporarily.

You will make it through despite it being harder than expected on your side of things. Always remember you can only control yourself and there is only so much you can do. Don’t stress yourself out too much when you have already taken steps to show that you care.

You

I don't think you're being selfish here.
If anything, the empathy you have about this hits you harder than it would a selfish person.
Being emotional about this is perfectly normal.

Your wife

At least tell your wife about the coworker not being able to make it there.
If my partner was in this situation I'd want to know, I imagine you would too if the roles were reversed.
It's not about looking for something out of it, sometimes it just helps to get things off your chest.

This trip

It's also perfectly reasonable to worry about your new immediate situation in this work trip.

I know what you mean, I'm a tech expert, not a sales guy. Customer sales meetings are usually inanely boring sales pitch with the occasional tech question.
Sales people talking together, with tech being an afterthought.
Your co-worker likely had some sort of sales pitch already prepared that you can reuse, but honestly, you don't necessarily have to become a salesman here either.
The meetings themselves will likely just be different, it's fine.
Just be honest and upfront about your sales person not being able to make it because of death in his family. Give them as much details as you feel comfortable with.
Customers are human too.

Firsthand experience, some of the strongest customer relations we have... were made the other way around: tech first, sales second.
Play to your strengths, get them interested, they can figure out the boring sales stuff afterwards for once.
It still depends on your audience obviously, but it's not necessarily a bad way to approach a customer.

You got this.

One thing I would like to tell you first: it's not selfish to want to talk with your wife about this. This is something that of course is affecting you and you should talk with someone about it. A great stepping stone to this is your post here, great that you're reaching out and asking for advice about this. I'm no expert in these things and have my own struggles just like you, but take one hour and one day at the time, don't stress it out. If you need counseling there is a few online services that you can use that has psychologists, one of the ones I've heard about is BetterHelp, I hope you're able to afford it. I hope this helps mate.

Sounds like your plan is reasonable: do what you can to keep business going. It doesn’t hurt to check in with your coworker about how he’s feeling. It can be by text if you want.

Yes, it breaks social convention for him to get emotional support from a coworker, but our society’s too dry in that regard to rely on the culturally-sanctioned channels for that support.

But also, it’s okay for your contribution to just be keeping everything running as smoothly as possible. That’s helpful too.

One thing I would recommend is to come up with your cover story about where he is. It doesn’t have to be a lie. You can just tell the truth that there’s a family emergency your coworker is dealing with. But even a simple response like that benefits from being worked out in advance, before you’re asked.

Also, since your own workload has increased, it’ll be good to pay attention to your own care. The one thing I would specifically caution is to pay attention to your hydration levels.

Being in a foreign country, it’s always handy to know where you’re going to get drinking water, and then to drink plenty of it. Being dehydrated will make you more forgetful, and right now you need to be on the ball with the driving and the contract negotiation.

I would just be as honest as possible about what you can and can’t do, and provide as much help as you can in understanding the tech. If you choose to propose a delay in closing, make sure to frame it in terms of everybody’s benefit, like:

“Despite the fact we want to move this forward, the reality is we’re missing a person that we planned on having present for this process. It’s not ideal, but it may be our best option to extend our closing deadline a bit. As much as I am committed to helping, and will help however I can, my vote is to delay this closing until we can make the best use of everyone’s input”.

However, the reality has to be dealt with that your coworker may be simply unable to contribute for a while.

Also, check in with us here in a couple of days and let us know how it’s going for you.

I think that if you tell the truth, and stay hydrated, you’ll be able to help cooler heads prevail and keep things managed in his absence. But I’m not kidding: pay special attention to staying hydrated as it’s easy to forget, easy to avoid, and makes a huge difference in performance in an unfamiliar environment.

Other people are giving you good advice about your emotions, so here is some practical advice for your customer interactions:

Be honest without being specific. Print out on a piece of paper in their language an explanation of your role, and that you are also covering your coworkers role who cannot be here due to a sudden death in the family.

"I apologize for any difficulty caused by the situation and appreciate your understanding and patience in these unusual circumstances."