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How much/how quickly is it leaking, is it external or internal leak, is it the original gaskets? If it's a slow, external leak, on the original gaskets then you have a really good chance of it working. I wouldn't try if you suspect a reasonable amount of internal leakage. If it's leaking a lot or the gaskets were previously replaced - chances aren't great. If it's an internal leak - it has zero chance of working. This is when I think it can cause problems. This might not be 100% accurate but for example - Normally coolant conditioner is suspended in solution and doesn't "react" until it's exposed to a leak. So it doesn't do anything to cause issues. In an improperly working cooling system the conditioner may be exposed to air pockets, exhaust gases, pressure/temp fluctuations that may encourage it to react internally where it's not suppose to. This may or may not matter or it might take more than one bottle to cause issues, I haven't tried that enough to say. But I'd probably try to diagnose how much/how quickly it's leaking and if it's internal or external before attempting it.

getting near 2500 miles now since repair so far so good.

if the codes are consistently in pairs like ; 1 & 2, OR, 3 & 4, that points to a possible coil problem because of 'waste spark' sharing of coil output. cetrainly oil on plugs or cable ends or bad wires, etc. could be an issue.

No way to know how long it will last. Might be 50k, might be 50. New fluid will help, but obviously it won't solve the problem. And it will still be a gamble every time you drive it. Make sure your never outside the towing range of your AAA membership.... Whatever you think it's worth (I won't get into that, the market is crazy right now), it will be worth a LOT less with a known major problem like that. Look at Impreza transmissions. 3.9 is a very common ratio for the Impreza/OBS in that vintage. They will likely have a slightly taller 5th gear than the heavier Legacy, but it's a pretty small change. Facebook marketplace and brand-specific groups are a great place to buy a transmission. I bought 2 Turbo (FXT and OBXT) 4.444 manual transmissions for $150 through a local group. One has completely unknown history, but still! I bet with a little digging, you could find a nice local transmission for 300 or less. Sure, it's a pain of a job, but if the car is in otherwise good condition, you will add thousands of dollars to its resale value. That's easy math if you ask me. You could probably find a shadetree mechanic doing side work in his garage and pay him to do it, and still come out way ahead.