yes, its pita that chris insists to check for libs and reaction classes versions manually
You have to. If you're using Reaction specific features that don't exist in 3.0/3.1 then it'll simply crash. You always have to request libraries of the right minimum version when writing software.
It's different, for example, when you're using the AmiTcp4 SDK while you're only using features that AmiTcp3 supports. Your software won't run with AmiTcp3 unless you recompile it with the AmiTcp3 SDK. NtpSync is an example of this. Works perfectly fine with AmiTcp3 when you recompile it (luckily it includes the source code).
However, in the case of Netsurf, it probably simply can't work on 3.0/3.1. Wouldn't be entirely surprised if it can't work with Aros either.