So much to say! Most important: this run was on PX68k so it is accurate for FPS & hardware slowdown. The hardware slowdown is the important thing! Obviously, this game and mode was balanced with it. If you try this using Windows EXE it will be much, much harder because there is zero hardware slowdown when have 100+ revenge bullets on screen lol @_@!~
I first tried Impossible using Windows EXE (like my Normal 2-ALL) so have quite a lot of experience with it... but eventually put game away because got very frustrated with the crazy # of revenge bullet.
When I switched to emulate on PX68k instead... wow. Really very big difference!
After switch, take around 10 more hours to achieve 2-ALL clear!! Also: the 2-ALL clear was achieved on the first attempt of reaching second loop. The main struggle is really the first loop to build score.
For me, the most important thing was: hold first life as long as possible on first loop. My run actually was very messy... hell a lot of shield loss during first loop... but I was very lucky to recover each one, and so only my first actual loss of life was only at Stage 0 on the first loop! This was very lucky! Actually, my goal for Impossible... was just to do 1-ALL... and I would be very happy. But I started second loop with only one life lost... so I immediately took it seriously... but this was my first exposure to it on Impossible.
Immediately, I felt second loop of Impossible was easier than first loop. I think... will even say it is easier than Normal second loop. Main reason is the revenge bullets are VERY clean and easy to read, because the bullet pattern used for them, is very good. My main complaint with Normal 2-ALL was the revenge bullet used the most common bullet sprite in game, so hard to read. Not the case this time!
Also: second reason is: because the revenge bullets in second loop Impossible are big and flashing... they generate more hardware slowdown. This is most easy to notice when they spawn, because the revenge bullet spawn on enemy explosion, so... you get a brief sudden "lag" when they appear because there is a lot going on at once on screen. This allows you to quickly read them, then reposition ship. So... even though the damn revenge bullet direction is random... but because there is now brief lag when they spawn, it is much more realistic to read and dodge.
Another important thing to learn for Impossible: is when to intentionally lose shield instead of using bomba. So, example: if I am now stuck and about to die, but I know my bomba stock is 3... and I know that next item carrier it is safe to do 3x power-up pickup... then I will choose to lose shield instead of bomba. Because that way... when I do 3x collection... I will get shield back, then +1 bomba. But if I had used bomba instead... then I will minus 1 bomba, and then plus 1 from item carrier... and will still be same.
The biggest struggle in Impossible... especially first loop... is very often, doing 3x power-up pickup is damn difficult. So it is important to know when it is safe to do it, so you can try rebuild resources. This is how I only manage to take first loss of life very late in the first loop X_X!~
I will definitely say... if you can clear 1-ALL in Impossible... then you can definitely do 2-ALL. One big advantage with Impossible is... it seems that enemy give more score. So... because of that... it is less important to do 3x power-up pickup. You can play 100% survival and only occasionally do 3x power-up pickup when it is very safe... and you will still score well enough to reach 7-8 extends stock by end of first loop. Then... after that... when going into second loop... you will be building more and more extends as both score and extends stock keep building up.
Overall... very highly recommend this mode! Thank you everyone for encouragement to do this... and especially suggestion to emulate instead of playing Windows EXE lol!! Wouldn't do this otherwise!!~
And finally: the damn Stage 4 boss (the many blimp that spawn one after another) is 100x easier on PX68k than Windows EXE. That was always my run-ender when playing on Windows EXE... usually will always have to use 5 bomba because the blimp shoot out 100's of revenge bullet and without hardware slowdown it is very hard to dodge... then would usually lose at least one extend here, then because ship power-up all lost will start Stage 5 in very bad position... and then just keep chain death from there. This single fight being easier was one of the big advantages!
Having poor Stage 5 after that blimp boss is also bad news because the Stage 5 boss have a million destructible parts which means whole fight have a lot of revenge bullets, lol. Fking revenge bullets zzz.
But yep, finally got the 1CC. I didn't think I'd get it honestly (always happens this way, huh?) - I'd been dancing around it for months - was relatively consistent with 5-stock 1CCs, had a few 3-stock runs where I lost it at the final boss, and even had a 5-stock 1CC where I finished with enough lives where it would have been a 1CC on default settings. 
At 8:45, I lost control a bit due to the helicopter, which meant I "had" to use a bomb—and consequently couldn't execute my clean, two-bomb strategy against the boss.
And with the extends being relatively fast to reach, there was no real need to score all that much just to get the clear indeed. 1bil on that other run is not bad either for "scorevival" purposes, my 1CC is around that too, even if there's obviously a lot more room for score improvements.
I might have to try and get the PS2 version as well, but I suspect it hasn't got any cheaper over the years.