An ale of those specifics should need about 150 billion cells for a 1 million/ml/°P. This is considered plenty of yeast for an ale. Many brewers/breweries use a 0.75 mill/ml/°P ratio would would mean 113 billion cells. So you're aiming for 113-150 billion cells for pitching into your beer.
Based on the krausen ring, and your timeline, the starter looks like it was close to finished. I'm assuming this started was not swirled/stirred because the ring looks pretty solidified; so it probably just sat still and did it's thing. The jar looks to be a quart but the half-gallon jars look about the same; I'll assume it's a quart jar. This would mean you used about 3 cups of starter wort.
Assumptions:
-Non-stirred/swirled starter
-0.75 quarts of starter wort
-Good viability on yeast
-Starter was complete when it fell
Using the above assumptions, and according to yeast starter calculators for growth rate in non-stirred starters, you may have reached around 130 billion cells; most of which were flocculated to the bottom of the jar. If you lost half of the cells then you would be around 65 billion cells which I would not consider to be enough cells. If most of the cells had flocced out and remained in the jar (i.e. you lost mostly spent wort) then let's assume 25% loss which would leave you with about 100 billion cells, which is still on the lower end of the range.
Just in terms of pitching rate, I would say your marginal for the number of yeast cells for a "proper" pitching rate. You would still make beer if you decided to just use your starter, and if you kept fermentation temperatures in check with good aeration then you may not notice a difference between non-proper and proper pitching rate in this beer. Lots of beer have been successfully brewed with that kind of pitching rate.
At this point, the choice is up to you on how to proceed.
As to contamination chances, this will rely solely on your handling of the yeast starter and if you think it somehow got contaminated before/during/after it's fall from your fridge. Contaminants don't fly into your starter, they have to fall into your starter so if it remained covered the whole time and it just leaked out around the tin foil then I wouldn't worry about contamination. You would want to be certain that you clean the lip of the jar using a sanitized paper towel (or something) prior to pitching.
How you proceed is very much up to you. If you don't mind a little chance, then let it ride and see what happens. If you don't want to take chances, then do something different.
I know this is somewhat vague but it really is a personal choice. I'm not certain what I would do. If I thought the starter was not contaminated then I might mix up a small amount of starter wort, add to the jar, give it a day or two, and then use it as is; then again, I might not.
So I made my yeast starter the other day, Wednesday. I put it on top of the fridge to keep it safe and out of the way, which proved to be more fatal. 
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