By "Church of the Immaculate Conception" do you mean the Basilica of the National Shrine of the Immaculate Conception? (There is a Church of the Immaculate Conception, but it's not generally a tourist attraction.) If so, be aware that the Basilica is on the opposite side of town from every other activity you've listed for 5/18. I don't think it combines well with them geographically, but if you're absolutely determined to see it, do it first instead of criss-crossing back and forth.

Thank you! Updated the name to Basilica of the National Shrine of the Immaculate Conception. Will put the route in the correct order :)

There’s no need to take a bus tour originating in DC to tour Arlington National Cemetery—it will cover things in the city that you will see at other times.

I would take Metro to the Arlington Cemetery stop, walk to the Visitor’s Center and then buy tickets for the HO/HO bus. The bus tour with stops to see the Changing of the Guard and Arlington House will take a couple hours. Yes, you could walk across Memorial Bridge to get to the cemetery, but it’s a bit of a schlep and you’ll be doing a lot of walking on this trip anyway. You might want to save your walking for when it’s necessary.

Also, remember that each if you must have a government issued photo ID (like a driver’s license or passport) to enter Arlington Cemetery.

Being that all Smithsonian are free go their website and see what interests you

Will definitely do the museums - planning for the rainy 05/18, just haven't decide which ones, but National Gallery and History Museum are my personal favorites.