Yes. It's easy. I've done it tons of times.
Oil - doesn't matter. Let me explain...
If you unbolted A/C lines on a brand new Subaru, removed the compressor, then reinstalled it again 4 minutes later - would you be worried about "oil" in the compressor?
Of course not - it's got whatever oil in it that is distributed throughout the system and will retain that when removed from the vehicle. Just like a part at a local yard. A reasonable mechanic or yard knows not to dump the oil out or expose the orifices to debris...they're often stuffed with plastic caps or something to prevent debris/humidity/oil escape when I get one if it's not removed directly from the car.
I'm somewhat overstating the illustration...but not by much - maybe time and conditions warrant adding some oil, and it is a good idea. Sure go ahead and do that - no it's not precise. You'd have to try really hard to get it wrong, you don't sound like that type.
Imagine adding up all the internal surface area of the compressor guts, air lines, evaporator, drier, etc.....there's gobs of square inches of surface area. Get it close. If you think you should add 1 unit of oil....and you add 2....it won't matter - it'll be distributed over such a large surface area that the small amounts we are talking about are inconsequential. If you didn't add any then all of the cumulative oil in the system will over time just redistribute itself into a slightly thinner layer covering the innards of your compressor (which has it's own residual oil already in it - they didn't dump cleaner down the thing!).
You can never known how much oil to add so it's always somewhat of guess. How many times has your A/C been worked on from brand new, how much oil has been lost due to depressing the valve for charging or testing over the years, has any of the oil "degraded", how many parts have been replaced...same goes with the "new" compressor -you can't cut it open and see how much residual oil and film is extant in the compressor. so it's one big huge guess.
Also there's no need to pull a vaccuum. Remove your compressor, bolt the new one in place. Very little air got into the system and what little humidity was in the "new" compressor will get absorbed by the drier. There's basically zero chance of all the cataclysmic nonsense people like to rave about online...it's crazy talk and practically speaking never happens to subarus unless you're a hack. Add 18-22 ounces - whatever yours calls for and you're done. If you live around exceptional humidity/heat or are transporting frozen goods in the cabin then pull that vacuum and get it perfect.
Yes - lots of shops//mechanics/DIY/HVAC people would not say anything close to what I just said....but practically and physically thats' what's happening. Sure - don't be dumb. Don't buy an unknown cheap compressor from some guy who had to unburry from the back yard garden pile and clean the bugs out of the holes - and dont' randomly guess on how much oil to add. Looks it up and make a very rough educated guess - you'll be right. By your sense of talking about this and detail and interest in diagnosing this - you sound like you know what you're doing and not about to just guess/hack/and roll dice here.
So yeah - it's easy. I've done it a ton of times. I've bolted on ancient Subaru compressors older than yours, some I've added oil, some I haven't....same result every time. They work and never need replaced again. But compressors here are cheap and easy to come by and I often have them on hand...so I get you that you might want to diagnose it more first and make sure it's the compressor if they're harder to come by there.