The gerundivium can appear without the esse complement as a adjective, just like the other participles (amatus, amaturus), and, just like them, it conserves its meaning as a verb, passive and future. (to be done, to be made, to be happened) V.g.: Delenda Carthago = Cartago must be destroyed = Cartago delenda est.
So that in "Et arva colenda dedit" colenda means "to be plowed" or "that should be plowed"
"Venus... filio se videndam obtulit" videndam means to be seem "Venus shows herself to be seem by her son"
They are not redundant, for she could have shown herself to be kissed, to be saluted or whatever: Venus filio se salutandam obtulit; se osculandam; se necandam
"Neque enim mihi fas.." Mihi here is dative of interest "it is not allowed (for me, in my case) to touch.." (perharps, better, I'm not allowed)
"Nec fas est te..." Te is subject of the accusativus cum infinitivo (It is not permitted you to carry...). So that you can't change one for another; but, perharps, you could introduce the tibi, or other dative, to indicate the interest, to whom the prohibition refers: Neque tibi fas est te hinc comitem portare Creusam.
As to the redundacy with haerebat, I pass, for I truly can't answer.
I hope I helped