My local store in southern CA said it's the same stock for both reservations and walk in. Each day at 8am local time they update stock numbers for reservations, so I'd keep checking. Basically if they have no reservations and you walk in, they will sell you what they have (or if you miss your reservation time, they release it for other reservations or walk ins).
If you're having a hard time finding anything in your area, I'd just order and have it shipped. Most Watches ship within a week or two (sometimes faster than quoted), so it's not a horrible wait compared to what April/May orders went through.
Overall you're correct- 38mm are much easier to get than 42mm. Same for sport vs. stainless steel.