Congrats on your pregnancy - your priority needs to be taking care of YOU, which in turn will mean taking care of your baby and your older kids.
I'm not sure why your niece thinks you should give her a crib - either say you need it for your baby or you gave it away when your boys were done with it. I think it's fine to say that your kids only had hand-me-downs, and so many things have been recalled that you just don't have anything left that's approved.
There is NO WAY that you should be expected to travel 400 miles while pregnant, for any reason. And where did this "couple hundred dollars" gift guideline come from in your family? That's nuts. I would send a card and either a gift card or something small - consignment shops often have new items that someone didn't want so you can save money but still send a token gift. Put a note in, if you like, saying how much this item was enjoyed by your boys and how you wish her the best during this exciting time, making it clear that you are thinking about her.
Just say you are not able to travel at this point in your pregnancy, period. If they're all fired up about your age, then use that as your "excuse" if you really think you need one. Honestly, the people who think we women dry up in our 40s are out to lunch. Kelly Preston just had a baby at age 48 or so.
You can either spend your time telling your family what you bought at consignment shops (making it clear that you have nothing for this 3rd child), or you can leave them out of the discussion entirely, whatever reduces your stress the most.
Perhaps your friends can put together a shower, and you can invite the relatives or not, whatever pleases you. The invitations technically come from the hostess, not from you - you just provide the addresses & stay out of it. Let's see how many of them travel 400 miles.
I'd ignore them and focus the energy on yourself. If it would help to clear the air, fine. But if it's going to stress you out, don't waste the energy.