Re: "A transaction cannot be ended inside a block with exception handlers."

I personally have sympathy, and if you submitted a patch to improve matters here I don't see anyone saying that it would be unwanted.

As for the circular dependency breaking use of deferred constraints (or your explicitly deferred triggers), you have the SET CONSTRAINTS ALL IMMEDIATE command:

postgres=# call do_insert(false);
ERROR:  tigger trg caused exception
CONTEXT:  PL/pgSQL function trg_fn() line 9 at ASSERT
SQL statement "SET CONSTRAINTS ALL IMMEDIATE"
PL/pgSQL function do_insert(boolean) line 12 at SQL statement
postgres=# create or replace procedure do_insert(good in boolean)
  language plpgsql
as $body$
begin
begin
  case good
    when true then
      for j in 10..20 loop
        insert into t(k) values(j);
      end loop;
    when false then
      insert into t(k) values(42);
  end case;
SET CONSTRAINTS ALL IMMEDIATE;
commit;
end;
exception
when invalid_transaction_termination then
raise exception 'caught invalid';
when OTHERS then
raise notice 'others - ok';
commit;
end;

David J.