Regarding the oil sump plug I had the same problem that it was stuck.
Broke 2 13mm sockets on it , also tried heating it up and in the end i used a chisel and really lager hammer + the gathered anger (due to the fact i was wasting 3 hours on the stupid sump plug) and got it loose by putting the chisel in the correct direction and hammer it loose but making sure not to damage the sump itself of course.
I got this tip from this forum.
Then bought a new sump plug + ring for 10 euro at the dealer.
Regarding the oil filter, the cap/adapter (which you put on the oil filter housing to get it loose with a socket wrench) needed for at least a 1.9 JTDM 16V (which i have) has an uncommon size and is hart to get.
The oil filter tool as posted above by Verbout is I think the best solution, even my local Alfa dealer uses it.
And indeed I bought a new one 2 weeks ago at the Lidl for 4 euro, so its also the cheapest solution.
The only thing is that by turning the oil filter tool by hand the grip on the oil filter cap is not enough so most likely you need pliers to rotate the oil filter tool tighter to increase the grip on the oil filter cap.
Next time I'm using it I will modify the oil filter tool by cutting a nut in half, welding both sides to the threaded top part of the oil filter tool so I can use a wrench instead to tighten it up, which makes life easier.