I am sure that many, including myself are rooting for you, Brickton, and are sitting a little on the edge of the seat as we follow your updates.

In my experience, sometimes you need to do a lot more than you were ever planning with an old car (or other esoteric project), before you reach that critical mass consisting of diagnostics, repairs and analysis, from which you finally reach your goal.

The key is to be doggedly systematic in your approach - something you seem to be doing well. For all the armchair MB diagnosticians out there, this is why your posts/thread/quest is a compelling draw, i.e. you are doing it the right way. You suffer, but its cathartic for the rest of us. Sorry.

As for HF compression tester, you can have it overnighted for not too much, or second dayd (I know, not a word) for less. The 14 piece HF kit is reported to work well on 617s and costs a reasonable $30 or so. Their 20% coupons are always on line. In fact I just bought the kit with a 25% off coupon I found on the internet.

If I were in your situation I don't think I'd abandon the thought of doing a compression test. The quick and easy repair or even a moderately strenuous one, both of those thresholds have been long passed in your quest, which is now something else entirely. That being the case the situation needs to be evaluated on its own merits ...and I think you are very close to reaching that "critical mass" I mentioned earlier.

Stay with it!