Is it as simple as jetting up the carb to run richer.
Perhaps a nice start, but "no"!

. . . . . the fuel system needs to be E-85 proof.
Very true!

I've read that "E-85" can be "up to" 85% alcohol but supposedly it varies quite a bit from station to station and maybe even batch to batch.
Correct! And a problem if your purpose was to convert to E85 for the "anti-knock" qualities, as this varies with the "mix"! Testing kits have been created, though I'm not well aware of their accuracy, this to be utilized with each fill-up to determine if a "tune" change might be in order; this both for detonation avoidance and A/F mixture needs.

You also need a way to deal with or prevent gelling caused by ethanol in the gas. 10% ethanol fuel gels bad enough after just a few weeks of inactivity. The gelling doesn't seem to affect port injection as much as carburetors.
Again correct! Fuel injection (as currently supplied by the O.E.M.'s) is a "sealed fuel system", this reducing the fuels exposure to the atmosphere. The biggest issue is that the alcohol, being hygroscopic draws moisture from the atmosphere and will continue to do so until saturated. Now, if that isn't bad enough, but then the alcohol is also highly evaporative, so with an "open" or "vented" fuel system it tends to dissipate into the atmosphere, but leaving the water behind and this becomes the headache stuff!
Also consider that if the fuel delivery system, pump(s), filter(s), fuel lines, possibly regulation and even the carburetor, is currently properly balanced for gasoline, it will prove "insufficient", this probably beyond just a "jet-change", for E85.

And then yeah, alcohol is lower in it's B.T.U. production in the combustion cycle, and why it requires significantly more by volume, so unless your combination would benefit from E85 in order to capitalize on a more aggressive state of tune, the net result could be less power!

Scott.