That's correct, it won't ruin them, but it will always produce an inferior product when compared to a roast correctly cooked to the temperature that someone wants in the first place. Even a home cook that properly dry brines and ages meat can make a better product than a restaurant meal if it's done with care, because they're working in smaller quantities for individuals whose preferences are already known. Restaurants don't have that luxury, which is why searing is the best solution for them. His kitchen is not a restaurant, and he's cooking for 10 people. Treating them equally well is not a big ask here.

I get the feeling from OP's replies that regardless of what is suggested here, he wants to make an unpalatable meal for his in-laws. There seems to be a lot of anger from previous meals that's coming out this year.