Disappearing coolant after overheating = blown head gasket?

[Eric]

Hi all. Thank you for an excellent site.
I'll be as succinct as possible...

Car:1998 318ti, 130,000 mi.
Engine: M44, BMW-rebuilt long block, 60,000 mi.
I've owned the car for the past 70,000 mi / 5 years.

1. I have read about others having a problem with no heat at idle.
I had this all the time I've owned the car, never thought much of it (I don't tend to use the car for stake-outs). It's better after bleeding.

2. Car has always seemed to have a slow coolant leak. Every now and then, I've had to put some more in - not often enough to recognize a pattern, and the temp never went above the middle line.

3. 2 weeks ago, I was snowblowing the driveway, moved the car to clear its space, left it idling with the lights on, so I could see (gets dark before 4), found the temp needle pinned when I got back to it after 20 minutes.

4. Like an idiot, had an irresistible urge to open the cap - after parking car and turning off, slowly loosened cap, allowing pressure to bleed off, then removed cap - Look, Ma, no coolant!

5. Filled with coolant when cool - put in as much as it would take.

6. Next morning, set off for work (ambient temp ~5F), car warmed up as usual, then just kept on going - needle buried in red after about 3 miles. Heat never came on. Stopped car, sat, waited. Scraped ice from inside of windows. At normal temp, restarted, headed home. Temp increased, went into red, stopped about 1/2 way into red and stayed there, drove home very slowly like that. Car stalled when I put in clutch going into driveway (creating perfect cork in 4 foot high snowplow mound). Took 256,000-mile diesel pickup to work.

7. Surfed internet at work, found out about little bleeder screw.
Went home, confirmed NO MENTION of little bleeder screw in owner's manual (in fact, it's even AIRBRUSHED OUT in the coolant filler picture). Got angry.

8. Refilled with 50:50 BMW antifreeze, bled air from little bleeder valve.
Repeated a few times after driving (found out little bleeder valve can be opened and closed with a quarter).

9. No more overheating, either driving or at idle, and heat came on sooner.

10. Then it didn't. Added more fluid. Then added more again. And again. Can easily add 1/2 gallon after 20 or so miles.

11. No steam from exhaust, no smell of coolant, no coolant puddles (never were).
Radiator smells of exhaust when you open it.
Compression, cyls 1 to 4, in psi: 180 140 170 190
#2 plug slightly oily, others normal, and about the same as each other.

Questions:
- Does all of this seem to add up to any specific pattern (head gasket, cracked head / block, imminent nuclear annihilation, etc)?
- I don't have a garage, and am not pulling the head in the snow in January in Maine. Is it okay to drive it like this, adding coolant daily, until the spring (like May) when I can finally work outdoors around here?
- Any other words of wisdom or humiliation?

ps: I'm a 45 year old guy who has owned and rebuilt LOTS of cars.
Little bleeder screw not mentioned in owner's manual.
I've never yet read a service manual before putting water into a car.
Little bleeder screw is matte black plastic on a matte black plastic background with the German word for "bleeder" embossed UPSiDE DOWN in tiny letters in black plastic around screw.
Am I an idiot? (you know what I want to hear...)