How can I let FamilySearch know of missing indexing?
As I searched Budapest IV 1934 marriages, I found 2 backdated marriages from 1906 and one from 1922 right below it, both were entered into the 1934 Budapest IV marriages!!! Someone looking for a 1906 & 1922 marriage records would likely never find either one of these!
Here's a link to the record https://www.familysearch.org/ark:/61903/3:1:S3HY-6XGW-MQ5...
How do I let indexers know about this to enter it
Indexing of the civil registrations is ongoing. (I'm pretty sure some of it is being done somewhere outside FamilySearch's indexing section, perhaps by computer, because every few months we get newly-published indexes for places that we never saw during indexing.) There is no mechanism in place for expediting or prioritizing any particular film for indexing; they'll get to it when they get to it.
Note that the two "out-of-order" marriage entries that you found are also "out-of-place"; both were "entered based on the foreign extract provided by the Interior Ministry". So if the family researcher checks the marriage register for 1906 Szent-György (presumably the one in Pozsony county) or 1922 Csáktornya (which was once in Zala county), he'll find the records in the expected place and time.1