Is this a correct usage of the conditional past perfect to translate “I would have had it! Without you always ruining everything!”?
Je l'aurais avait! Sans toi toujours en train de ruiner tout!
No. As I see it it could be what would say a villain seeing his plan failing. He could say:
"Je l'aurais eu, sans toi à ruiner toujours tout !"
Splitting the sentence is weird in French, keep it as a single sentence.
"Je l'aurais eu, sans toi à toujours ruiner tout !"
Is correct too, but sounds less like the nasty guy whining, and a bit weirder too.
I would change the previous sentences slightly.
I'd much rather say "Je l'aurais eu, sans toi pour toujours tout ruiner !"
So "pour" instead of "à" and the order of the next words makes more sense.
As for "Without +pronoun verb +ing" it translates to "présent du subjonctif" e.g. "Sans que tu ne fasses" ("Without you doing") -> Sans + que + pronom + verbe (au présent du subjonctif).
In french i would say
"Je l'aurais eu si tu n'avais pas tout gâché, comme d'habitude."
I am not english native, then I might have misunderstood your sentence. I understood "Without you always ruining everything!" as, if the subject used to ruin everything he tries to manage
If "Without you always ruining everything!" means, that the subject ruined this business in particular, simply write :
"Je l'aurais eu si tu n'avais pas tout gâché."
"Ruiner" is an old use to translate "to ruin", moreover if the speaker is angry or excited. You can also replace
gâché (gâcher) with
bousillé (bousiller) or
pourri (pourrir) in a familiar use, or with
anéanti (anéantir) or
saboté (saboter) for a more formal use.