Leni
“what’s to keep this spruce goose from getting shot down the minute it crosses any border”
I am sure it has some surprising countermeasures, but the biggest countermeasure will be to fly high and away from the actual targets.
SAMs are very expensive. Iran (for example) is a big country.
Lots of Iran is unguarded. Plenty of known holes in radar coverage, too.
The SAMs that can reach out and touch an Etan at 40K feet are generally clustered around the high-value targets.
You just fly around the tagets, but close enough to get pictures. (Basically, edge of horizon from 40K feet.)
A normal country would have plenty of fighters to get these slow beasts, but Iran purged its military of officer ranks, as they were seen as loyalists to the Shah (many of whom trained in Israel and USA, in fact). The Iranian airforce (then, the second best in the Middle East) has never really recovered.
Also, these would be part of a package -— you’d have stand-off cruise missles coming in to hit radar and missle batteries first (possibly from an Etan).
Then you’d come in and start bombing the actual targets, using F-15 and 16, together with medium range ballistic missles.