Bad Manners and Brimstone

I'd contact the host and let them know that you would like to attend, but are also concerned about covid-19.

You can ask what precautions they plan on having. Will everyone be wearing masks? Will there be space to spread out so you aren't close to one another? Will the festivities be outside or inside? If the host doesn't take your questions and concerns seriously, you will have a good idea that there won't be many precautions taken.

Let them know that you'll have to make your decision much closer to the actual date, because you won't be able to attend unless infection rates have gone way down (as we all hope they will).

You can also ask for the best address to ship a gift in the event that you can't attend in person.

If you have particular concerns (like you are at risk or care for someone who is, or you are more likely to be a carrier because you are working in public), you could mention that. But you don't have to give an excuse to follow good pandemic infection-control rules so don't feel like you need to justify your concerns.

Normally, most of this would be considered fairly rude. These are not normal times.

I agree with the above.  Contact the host and tell them what you told us.  August is a good ways off but you never know how these things work.  Better safe than sorry.

It is an interesting topic, too! Nice to have an etiquette one.

Some other answers to specific questions, at least, my own thoughts:

Is it appropriate to wait things out to observe conditions prior to making a decision?

   Yes. In fact, I wouldn't even tell them I was waiting to decide; I'd just wait. I'd plug in a reminder on my calendar for a week before the event, and then I'd decide them, and RSVP.

Should I assume that a mask will be required?

I think you should assume that any official restrictions or recommendations will be honored. And that all guests should be prepared to wear a mask.
   If you don't want to attend unless other people ARE wearing masks, that would be a question I'd ask the hostess before deciding: "It's important to me to be safe, so I'm wondering if you know whether people will be wearing masks."
    If she says no, you can decline at that point.
    If she says she thinks so or doesn't know, you can decide if you want to risk it. And if you arrive and there are not enough people wearing masks to keep you comfortable, you can give greetings at the door, wave a cheery goodbye, say "Sorry not to stay--I'm taking extra precautions, so I'm going to leave, but I hope you have a great time. Happy baby/wedding!"

(What if I am wearing one and others are not?)

You can decide whether you want to leave or stay. I'm torn on whether you can ask people to wear masks. My vote would be to just leave instead; let them live with the consequence of not wearing the mask, and see if other people will make your point for you. (Sort of like, if you post something here, and people are arguing with you, it's a good idea to just wait and see if others will take up your cause.)

If rates spike closer to the party is it appropriate to cancel (guest or host)?

Yes. Because safety trumps etiquette. Even before a sometimes-deadly, often-unpleasant easily spread virus, you could cancel because you are unwell.
    You don't have to go into a lot of stuff; you can just say, "I'm not feeling well, so I won't be able to attend." If you want, you CAN say, "I'm going to drop off the gift and wave from my car; I want to stay safe, and the surge in cases has me cautious."

It is an interesting topic, too! Nice to have an etiquette one.

I try!    I love hearing how different people view different situations.

Always such great, in-depth thoughts, Hmmm!

I had not even thought of the eating/drinking with a mask issue. How awkward and strange. I was more concerning about trying to converse with Aunt Gertrude who is hard of hearing to begin with!

I certainly hope that we will continue discussing, but I am thinking that I will not be attending. One of my jobs just contacted me and we will be returning to working onsite in July. The day before the shower is a planned event where I will likely be in contact with 100 (or more) people throughout the day. Unless the virus up and disappears, I think I would be bringing too much risk for the MTB and guests. Heck, I am not sure how comfortable I am with being in contact with so many people!!! I don't think it's fair to subject the others to that.

I just saw a news segment talking about how to begin to expand your social interactions. One of my big take-aways was if possible do it outdoors. I would probably be more comfortable if the shower was held outside. In my part of the world, August has many nice days, although I would worry about heat (some days get very warm) and potential storms. A morning party might be less hot and may avoid the pop-up afternoon storms. October has a few nice days that would allow for an outdoor party, but many would be uncomfortably chilly. (We even get a bit of snow towards the end of the month.) But an afternoon party towards the beginning of the month may work out. But really you are always throwing the dice as any month can have a day that an outdoor party would not be feasible.

Being that your party has multiple hostess, I am assuming that the guest lists isn't small. Could you divide them up into different parties? I see issues with this, but just an idea to complicate things for you!