This is just a theory, so apologies if it doesn't work, but you could try the following:

/castsequence blah blah blah
/stopmacro [harm, nodead]
/assist Party1

Of course this relies on /stopmacro allowing you to use those conditionals with it and the castsequence not bugging out because of it.

No idea if this would cause a problem with your healing set-up either.