Minimal training for HIM 2 workouts/discipline/week?

Swimming can have a some variation between people in a situation like this. Best would be to get in the water a few times and see how fast it comes back. Or at least estimate a satisfactory amount may. If it does so quickly, great, hold off some more. If not, then be glad you started earlier. I had some ok speed 2 years out of the pool, but would tire & get sore much more easily.

Run more often than that. Volume comes easier with more frequency too. That's part of what was given when you were around before too (I remember some of the discussions then).

@ OP - You'll be fine on this lower-volume approach. Just adjust your expectations accordingly, and I suspect you'll actually have a very successful race day.

I wouldn't recommend this approach for beginners, but you're a Cat3 (which is easily FOP by triathlete standards), and an admittedly capable swimmer, so all you have to do is hold most of your bike ability and get to swimming the 1.2mi comfortably (which should be pretty easy a level to get to on 2 swims/wk given your background), so it's really all about training the run for you.

Seriously, you could even consider backing off the bike to 1 time per week with a combined harder/longer type bike effort, and run 3x/wk if you want to given your bike background. 

I'd also caution against going to hog wild doing a lot of HIIT running on this low-volume plan; I know you hear a lot about subbing in intensity for volume, but that tends to be safest when you're either doing nonimpact (swim/bike), or have a big running base to support it. Be very careful with the HIIT run training - I myself would postpone it completely until I felt I was running my target weekly run volume no problem even with multiple short speed pickups in there, and then I would be VERY careful about the HIIT. Meaning the moment you feel worried about overdoing it, you pull the plug on it and save it for next time. 

But yes, I do believe if done properly you can def do decently well, if not surprisingly well even at HIM distance with 2x/wk workouts for SBR. Remember  that consistency is the key to success - a lot of folks who train for 4 months CONSISTENTLY with this 2x/wk SBR without interruption or setbacks, will outperform folks with a planned heavier training regimen but who can't sustain it for the full 4 months and then have intermittent setbacks/injuries and days/weeks off in there.

Oh hi! I remember you!

Listen to brigby and yaz. They've got the goods.

There was a fat guy here who'd done a couple sprints before, then did a HIM on ... zero. Sure, he couldn't feel his crotch for 6 weeks afterward and he hasn't done a tri since ... so ... you COULD do the same, but really shouldn't. Honestly, it's not so much the minimum you could do it on -- because that would be zero -- but how you will fare in the days following / recovery. Life will really suck if you can't be there for the kids or show up to work because you're in agony for a week.

You've got a really decent base. (Everything I say is predicated on that). If you're only running 3x/week, all easy. (Look, you could WALK the HM part in well under 3hr.) That said, I'd rather ship someone off to a HIM on 4-5 easy runs/week totaling 1:30 - 2:00 than 3x/week (same time / distance, occasionally with a longer run). Remember you can run around the block in business shoes for a 10min run (you can run in bike shoes for 10min off each run, and that's a lot like running through transition). They add up. Ultimately it's the total you've done come race day, not "key" workouts or longest rides/runs.

You're also in luck because running transfers better to cycling than vice versa, and you're pretty good to go there.. Unfortunately nothing translates to swimming ... cue brigby again ... and remember you can toss in 10-15min of deep-water running after each swim. 

It's well past midnight. I've been up since 5:30am. You should therefore probably disregard everything I say ... but welcome back