Oh man, you have been having "fun" haven't you!
First, and most obvious thing to say is yes, I think a little bit has broken off the end of the circlip on the end at the bottom of the picture - but it's not much so I'm not absolutely certain. Can you see if the end looks as if a bit has been sheared off? The clips I've seen have always had squared off ends - no sign of the dimple effect I can see on yours. A new circlip doesn't cost much so if I were you I'd buy a new genuine Fiat one and compare the two. If a bit has broken off, the question which would then be bothering me, if I were in your place, would be, Has this just happened or has it been done on some previous removal of the shaft? To be frank there's not much you can do except shine a bright light into the "hole" where the shaft came from in the hope you might see the broken bit in the splines of the spider gears. I think this is unlikely but worth a try eh? If you do see the bit don't poke at it with a screwdriver. Sod's law dictates that you'll almost certainly dislodge it so it drops out of sight! Instead get one of those wee magnets on an extending wand (like a car aerial), works a treat. If you can't see it looking in "the hole" you might find it will come out if you pull the drain plug? The drain plug on our Panda and my boy's Punto is located at the back of the diff casing so very near to where the bit might drop down? probably as well to do a complete oil change on the 'box anyway? If you think there is a broken bit and you can't find it then what do you do? I don't think I could face dismantling the box. I might try propping the box up so the drain orifice is at the very bottom and pumping a gallon or so of paraffin or diesel in through the filler hole to try to flush it out? and I would try to get a magnetic drain plug for it - I've seen "universal" ones on ebay - in the hope it would retain the wee bit. In fact I've been thinking of getting magnetic plugs for both of ours anyway.
I'm sorry, but it's really easy for me to say now that where you went "wrong" removing that inner joint was in applying a constant gradual force to it. I've removed many of these sort of joints - held in by circlips - on a number of different makes of vehicle during my life and what they need is a very sudden application of force - a "jerk". I have a long tyre lever, it's about 2 ft long, and I get the end of it under the edge of the inner joint with the flat of the lever against one of the gearbox bolt heads - so there's a very considerable mechanical advantage going on here - and give the handle a sudden, sharp and vigorous pull. Usually the joint just pops out but occasionally it doesn't and I think this is because the circlip is not lying evenly in it's groove. If it doesn't pop out after a couple of vigorous snatch jerks then don't persist. Instead give the joint a wee wiggle or tap back into the casing so as to free up the circlip and turn the shaft about a third of a turn then try the tyre lever again. I find often the joint which was previously just not going to move will now almost leap out on it's own. If it still won't budge just repeat the process and try another third of a turn. If you get REALLY VIOLENT with them when they won't move all that happens is the circlip digs into the splines and life gets a whole lot more "messy" - especially if you go really "APE" and crack the gearbox casing! Sometimes using 2 levers, if you can get access, one on each side, works well. The "trick" though is to apply a sudden jerk, not a steady pull and "rocking" it with the two levers sometimes helps too. The big thing is not to allow the splines to dig into the circlip.
You should be able to get a new circlip relatively easily but be careful as they come in different thicknesses. I think a genuine Fiat one obtained from the dealer is unlikely to cause problems but if you source one from an engineering suppliers you'd probably be better to take the old one with you to compare sizes.
You're making progress though which is good! So don't loose heart! I've lost count of the number of times I've found myself lying under a vehicle I really care about, like one of the old Hillman Imps I was so passionate about, thinking to myself "whatever was I thinking, I should never have started on this" whilst perhaps blobs of red hot underseal drip onto my chest from the welding I'm doing or maybe a subframe retaining bolt snaps off inside a captive nut on a box section to which I have no way to access the other side off! Oh the joys, the joys!