I got at this thread via a Google search on Wyeast 2124 and W34/70 and read thru it.
I reckon many others will get here in a similar fashion, so, that's why I'm 'reviving' the thread as Fermentis itself has recently published a study to W34/70 (here) in which they found that W34/70 can ferment...
Well, normally you would take regular gravity readings towards the end of fermentation. If the gravity stays the same for two, three days in a row you can safely assume fermentation has finished. Don't try to 'read' the air lock.
Now, hop creep is a slow process and you wouldn't notice it...
In that case I'll bite on #3:
Did the calculations for you and the numbers you got are about right. They differ ± 1 max, so, that's possibly a rounding difference.
FG does depend on the yeast used; mainly on the attenuation of the yeast.
US-05 (att. 81%) will give you an FG of around 1.009...
Batch priming is a good thing to do, but... it requires moving the beer to another vessel in which you can prime it and then bottle it. Not only do you need extra equipment for that, it also takes time. And there's the risk of oxidation due to the extra contact with oxygen.. that's unavoidable...
Hops harvest time typically is between mid-August and end September. Not surprising you can only get US hops from 2019; most of the 2020 harvest is still out on the fields...
What would be surprising is if your Cascade for your APA is from 2020 – it'd be either a very, very early harvest...
You could even contemplate putting one or more pizza stones in the oven; they're great for holding temperature in ovens. Especially if you have any make sure you put 'em in!
Did you sparge after the mash? The low OG could easily be explained by not sparging. Lots of sugars then remain in the drained mash..
Or, if you did a BIAB and the recipe was for All Grain, did you boil off the excess water? You're OG after mash will not match the recipe's predicted OG if...
Why choose? Just do them both in order.
I'd do #1 first – it's the simpler of the two because of the dry hopping in #2 – and then #2. You're going to need a recipe for Batch #3 after you finished Batch #2 anyway 😉
Just be careful with the oats if you brew in a Brewzilla or similar apparatus...