Unconfirmed transactions after many days of the payment (does this even happen)?
there are many ways you can have a transaction not-confirm for a long time or even ever!
for example paying anything between 0 and 1 satoshi/byte means your tx won't be relayed by majority of the network and as a result it will not be confirmed.
other cases would be creating a long chain of unconfirmed transactions. in simple terms you send coins to address A then from A send to B before previous tx confirms and then from B to C, from C to D and so forth. it has happened with some services like exchanges and sometimes lack of propagation of children means long confirmation times.
A transaction which showed confirmed till 4 or 5 blocks but then was 'reversed'? (if I read right, after 3 confirms this is rare but possible)
in bitcoin the cost and chance of something like that happening is so tiny that it is considered to be impossible.
as long as a transaction is not confirmed you can double spend it. whether this was a scam or not is not possible to guess by only looking at the link.
ps. if the tx is from 2015 then it can't be RBF as mempool was not close to being full back then and also BIP-125 was introduced in December 2015.
Afaik "reversing" a transaction happens only if the block which included the tx got orphaned.
This means that soon enough nobody will have anymore that block, so there will not be any record to show you.
an orphaned block doesn't necessarily mean "transaction reversal", when a block is orphaned it means there is another different block that most probably contains the same exact transactions in it.
"reversing" can only happen if there is an attack where the attacker first spends the transaction and have it mined in block X then with his higher hashrate creates the same block and instead of mining the same tx, he mines another tx that spends the same output but creates different outputs.
the cost of this attack (51% attack) is so high that it requires that tx to be huge (worth millions) and people who accept such transactions amounts won't accept them with only 1 confirmation. and since reversing a tx that has more than 1 confirmation costs a lot more and it increases with each confirmation, this attack becomes nearly impossible.