Python 2 by Wheat Wizard (fourth iteration)

import sys
if set("".join(open(__file__).read().split('\n')[4:]))-set(' &)(,.:[]a`cdfijmonrt~'):sys.setrecursionlimit(1)
for m in sys.modules:sys.modules[m]=None
del sys;f=lambda\
c,d:(`int([]in[])`[:[]in[]]).join([((c)and`dict([((int([]in[])),(int([]in[])))])`[(int([]in[[]])):][(int([]in[[]]))].join([`dict([((int([]in[])),(int([]in[])))])`[(int([]in[]))],`dict([((int([]in[])),c)])`[(int([]in[[]])):][(int([]in[[]])):][(int([]in[[]])):][(int([]in[[]])):]])or`int([]in[])`[:[]in[]]).format((int([]in[]))),((d)and`dict([((int([]in[])),(int([]in[])))])`[(int([]in[[]])):][(int([]in[[]]))].join([`dict([((int([]in[])),(int([]in[])))])`[(int([]in[]))],`dict([((int([]in[])),d)])`[(int([]in[[]])):][(int([]in[[]])):][(int([]in[[]])):][(int([]in[[]])):]])or`int([]in[])`[:[]in[]]).format((int([]in[]))),`(int([]in[]))`]).rfind(`(int([]in[]))`)

Try it online!

No exploits, just a function to add using only characters ' &)(,.:[]a`cdfijmonrt~', as intended (actually only '(),.:[]`acdfijmnort').

I made no attempt to make it short; I just wrote subexpressions for intermediate values like 0 and the empty string and string-substituted those in. Explanation to come...