Is it actually worth it to research that?  Maybe if you were defending an important planet and had 5+ hangers (15 squads total).  Researching the 2nd level would give you 30 more strikecraft, which seems significant.

In entrenchment, I believe the upgrades to the hangars, ie flak turrets, shield bestowal, phasic trap, make them worthwhile in certain situations in MP and in more situations in SP. I witnessed one of the DT guys defend himself with a Starbase, 1 Halcyon, 3 repair bays and  a couple of hangars against 2 very skilled players who spammed bombers once they saw the starbase. All the while, the DT guy was able to build an assault fleet at planets he had take towards the center of the map. Personally I've witnessed a fleet supported by hangars and starbase thwart my bomber attacks for a long, long time with a combination of fighter suppression and flak turrets on the hangars, despite being vastly outnumbered. They are powerful defensive tools, and I no longer take them as lightly as I used to. Yes they are not as versatile as carriers, but they have more health and better armor, and they dont take up fleet supply (where as carriers take up quite a bit), which can be important when you're on the defensive. I'd say hangars are definitely useful in SP, and situationally useful in MP (particularly in the suicide spot).

Question: is Shield Bestowal worthwhile? I get it quite commonly in singleplayer, but of course haven't touched MP.

It's also good for keeping starbases alive for that much longer. If you can actually get turrets, a group of those with the Synergy ability buffed by the Hangar take forever to kill.

Me too.

Probably, I don't remember, it was a couple of weeks ago. I do remember that after the bomber spam 2v1 failed, the opposing team gave up and GG'd it.