Multi-instance support is probably good and useful thing, but I do not think it should be done this way, i.e. reinstalling with parameters or dialog options.
We must not forget about silent install that is used by most of distro-specific packages (if not all).
I.e. to support multiple instances I prefer a separate script controlling instances: create & drop support.
Install into non-default path is always treated as 'secondary' install with following limitations:
- links to .h files in /usr/include and libraries in /usr/lib are not created in order not to overwrite primary firebird installation
- init.d or xinetd startup files are created with a name posfixed with installation path (if one uses -path /usr/local/fb4 startup file is named firebird.usr_local_fb4)
- services are disabled by default (i.e. user must start /etc/init.d service manually or enable xinetd service manually)
By default - no, this can break distro specific builds when utilities go to /usr/bin, server - to /usr/sbin, etc.
But you are right - presence of binreloc should be detected and in absense of it switch -path should be disabled.