Are you sure it's paranoia? I would never use use engine flush on an Alfa and i have just read a thread of a member whose engine let go after he did an oil change and used engine flush.
The people i trust on this forum would never use it on an Alfa either.
As for using synthetic he should have been using it all along? I would use FULLY Synthetic rather than semi synthetic, its a bit more expensive but worth every penny.
Yes I am sure it's paranoia, having used it on several 16v engines in the past without the slightest problem (one of the said engines racking up 190k miles before i sold it), having spoken to mechanics that use it every day,.... if really worried by it the best method would be to flush, fill with cheap oil, run for a few miles, drain and then fill with synthetic.
The issues that cause panic as far as i'm aware are that-
1.Flush could dislodge a large chunk of debris that would subsequently block an oilway and lead to failure, not very likely as .A. the oil filter should catch it. .B. the engine would have to be in a very poor state for such debris to be there in the first place and the likelyhood is that the would dislodge flush or no flush.
2. On refilling the engine with new oil a residue of the flush remaining in the engine would contaminate the new oil and break it down, not very likely due to the flush being volatile and also massively diluted in the new oil but to be on the safe side perhaps use a cheap oil first (as above) for peace of mind.
I would need to see evidence not only that the engine failure you mentioned was undoubtedly caused by the flush, that the flush was correctly used etc before condemning flush as not being suitable for ALFA engines, ALFA engines being pretty straightforward run of the mill mildly tuned 2.0 16v's, nothing extraordinary or magical about them, if anything the general consensus seems to be that they are quite tough.
There does seem to be a tendency on this forum for someone to provide some annecdotal evidence and then for eveyone else to run around like headless chickens believing it
As for fully Synthetic being expensive, i'm not sure it is anymore,i.e. alfatecnico at sutton in ashfield will do you a change using shell helix ultra fully synthetic for 50 quid inc filter.