Is "heinz sight" an eggcorn of "hindsight"?

No. Quoting Wikipedia:

[An eggcorn] introduces a meaning that is different from the original, but plausible in the same context, such as "old-timers' disease" for "Alzheimer's disease".[1] This is as opposed to a malapropism, where the substitution creates a nonsensical phrase.

There doesn't seem to be any plausible, sensical reading for Heinz sight in this context; so it's much more a malapropism than an eggcorn.

(Mark Liberman's original post gave a somewhat different classification, describing the contrast with malapropisms as one of pronunciation:

"egg corn" and "acorn" are really homonyms (at least in casual pronunciation), while pairs like "allegory" for "alligator" [...] are merely similar in sound

This would place Heinz sight as at least a borderline eggcorn. But subsequent usage seems to fit the classification Wikipedia gives much more closely than the original one.)

Yes. Quoting Wikipedia:

[An eggcorn] introduces a meaning that is different from the original, but plausible in the same context, such as "old-timers' disease" for "Alzheimer's disease". This is as opposed to a malapropism, where the substitution creates a nonsensical phrase.

It is not a malapropism because (to paraphrase Language Log) hind sight and Heinz sight are really homonyms (at least in casual pronunciation).

And the phrase makes sense, so long as you accept that Heinz Sight is a thing Heinz make to improve your sight. Just as much sense as oaks coming from eggcorns.

One possible way to turn this into an eggcorn is to rewrite as

I'm fairly sure in Hein's sight he perhaps regrets the decision.

Heinz then is probably a fun way of writing Hein's.

A summary from Grammar Girl:

• Spoonerisms are what you get when a speaker mixes up sounds, making phrases such as better Nate than lever.
• Mondegreens are what you get when listeners mishear words; for example when people think the song lyrics are Sweet dreams are made of cheese instead of Sweet dreams are made of these.
• Eggcorns are what you get when people swap homophones in phrases, such as spelling here, here H-E-A-R instead of H-E-R-E.
• Malapropisms are what you get when someone substitutes a similar-sounding word for another, such as He's the pineapple of politeness instead of He's the pinnacle of politeness.

I agree with Optimal Cynic: heinz sight is a malapropism (although it sounds like something an Induhvidual would say -- which is probably the very definition of a malapropism...)

(Most famous mondegreen ever: "Excuse me while I kiss this guy.")