Both habuerō and lēgerō (not legerō, as there is no such form) are future perfect forms.
A very literal translation of the end of the quote would be:
Sī unquam habuerō codicēs saeculārēs, sī lēgerō, tē negāvī.
If I will ever have had secular books and if I will have read them, I have denied you.
That is, Jerome can only possess and read such books if he ...
It is the second option with arbitrata.
For the purposes of agreement, you can think of the participle as an adjective, so that Syra arbitrata est and Syra Romana est have exactly the same form.
The gender agreement holds for all subjects of all numbers in the same way.
For example, uxores arbitratae sunt and mariti arbitrati sunt.
And there is no difference ...
I found two examples of it followed by a vowel but scanning long in all of Catullus, Ovid, and Vergil.
Here are examples with context to show that perfect is likely:
Cuspis Echionio primum contorta lacerto
vana fuit truncoque dedit leve vulnus acerno;
proxima, si nimiis mittentis viribus usa
non foret, in tergo visa est haesura petito:
longius it; auctor ...
I've read the following etymologies
-ī> Directly from the first person perfective ending *-h₂e of PIE. (from Origins of the Greek Verb by Andreas Willi, Pg 8). He states the PIE *-h₂e became -ai in Old Latin, subsequently evolving into -ī.
-isti> Probably from PIE *-s-th₂e . The /s/ aorist marker was attached to the stem, followed by Stative *-th₂e, ...
Your question gives me the impression that you might be confused about the following points:
The perfect passive infinitive of "amo" is formed with the infinitive esse and a form of the perfect passive particicle amatus/amata/amatum. Any perfect passive participle inflects like an adjective of the first and second declension, with a masculine ...
I don't think it's 100% right to translate infinitives as participles, as you're doing. Esse is less "being" and more "to be," and the difference isn't trivial. Esse, for example, cannot be used attributively, whereas participles can. It's the difference between étant and être.
For your specific questions, yes, in English the perfect ...