The term you're looking for is "crypter". Heuristics may be more sensitive to the executable-packer-like behaviour than the crypter itself. Signatures which look for crypters will obviously target constants if they're around.

Speed is not a factor - size is. Cryptographic security is far less important than fingerprint evasion: rolling your own is not therefore as insane as it usually sounds.

RC5 is therefofe a slightly odd choice. Block function's a small Feistel, but the key schedule is a pain which totes constants around, which is poor especially from a polymorphic/metamorphic crypter's perspective, because that means you'll have to calculate them or include literals, and they're distinctive constants which make it pop out.

AES has a similar issue with the distinctive S-boxes - include or calculate them.

RC4 variants are more common (but you need scratch space), as are smaller more native stream ciphers, right down to good old XOR-based crypters, which will do the job. RC5 is just not the first thing that'd spring to mind to me.

It obviously was what sprang to mind to the author of that particular bit. Code styles differ, as do skill levels. Big team.

They speak English. It's big. They're interested in Ireland. US or UK is a good guess. I don't think this matches the Detica one for GCHQ, so my early suspicion still remains NSA. We're missing huge chunks though.