Ok, now, first of all, you gotta tell me, are you talking about macross or robotech? Coz right now, you're mixing the whole thing: the macross series, movies, and robotech.
point 1:
regults, as you said, are zents foot soldier weapons, they're not anticapship weapons

can Valkyries really threaten a sent capship? No, that's pretty obvious all along the series. Really, erread your sentence: "If the Zentraedi footsoldiers can't threaten warships than what good are they?" Well, if they can, the zents wouldn't really need anything else

point 2:
ok, remove the pinpoint barrier ( which is found thanks to an anomality from the fold engine, it's not a default system of the SDF1)
Then you get a hull that can be pierced by about anything, including crashing regults, or just a zentraedi female armor pulling on it ( when mirya drops the 3 spies in the ship ). Not super tough, heh...
As for the weaponary, most of the SDF1 weaponary is shot from ground mechas, like spartans and others. remove all that, and the SDF1 is naked.
As for Dolza's ship, of course, it's shooting from the inside, d'huh

as for the barriers, as I said, it's not a default feature of the thing, but fine: ( you're not lucky, my bro brought me the test DVDs for the collector to be releasedso I've actually seen half the series yesterad**day and this morning

) the pinpoint barrier can stop individiual shots fairly easily. but it's brought down when khyron or whatever it's spelled decides to "fool" britai: he's supposed to just frighten them, but he orders and old ship of his fleet ( "old fleets are pron to being inaccurate" ) to "accidentally" shoot at the SDF1. That's that lone, old crappy ship, with one single beam "seriously" focused on the SDF1 (' that's the first time a capship really shoots at it ), that takes down the barrier. I repeat, ONE SINGLE BEAM FROM ONE SINGLE OLD, CRAPPY SHIP. Your exemple was well chosen

thiod point: well, nothing more to add.
I'd also say that during the show, in some scenes, the characters cross the screen or jump above something in 3 frames, do you really expect that every single turret is depicted right?

The SDF1 obviously fires with everything it has, when it's attacked. You just don't see it.
finally, I've never said the ( original ) SDF1 was inferior, I said it was not superior. As for the main gun, well, seems that's the only part in the ship that never caused any problem ( the antigrav engines messed up, the fold engine "disappeared", the pinpoint barrier cut the power to the main gun ).
Anyway, I don't need to convince you, watching tyhe series should be enough for you to understand all that, hell, just listen to Gloval, he keeps saying that the zents are playing with them and stuff like that.