What Time in the day?

This is correct, but you can still rent an airbnb with up to 12 people. I don't think the ban on events would apply to OP because it's so small.

Where you live will determine how strict the rules get but I highly doubt you will see this happen: all persons must wear a mask outside unless on private property. I live in a state with strict COVID rules, thankfully, and they never got to that level. That said, weddings in parks usually require permits (even without COVID) and gatherings in parks were definitely not permitted here until recently.

You should check with your local parks department and see what steps you need to take to hold a wedding in the park you want. Then for the brunch bit, again your local restrictions will guide whether that can be out at a restaurant or if takeout would be a better option.

I have the same threshold for exposure that your mom does, so for me, and her, it wouldn't matter at all if (extremely unlikely) federal restrictions that you fear were enacted. Your mom needs a place to eat out of doors. So if you want her there, you need to plan for that, regardless of increased or decreased restrictions by April.

Looks like you have the park thing figured out (no permit needed), so then you just look for a local restaurant with an outdoor patio, or do a picnic at the park. Then the time of day would be entirely up to your choice, your officiant's availability, and your choice of restaurant vs. picnic.