[Exim] Controlling the number of exim processes.

I've been spending most of today trying to understand the various ways in
which the number of exim processes running on a system can be controlled
in an effort to put an upper bound on it. The system I'm looking at is
slightly underpowered and is prone to load spikes.

This is complicated by the fact that most of the delivery is to downstream
intermittently connected systems which are using ETRN to trigger mail
delivery.

As far as I can tell, I can control strictly the number of processes
running from queue runs by using 'remote_max_parallel' and
'queue_run_max'.

However each ETRN request implictly starts up another exim process, and
since these messages may have recipients other than the domain being
passed, those processes can persist attempting those other deliveries for
some considerable time. [Several hours at least]

1. Is there a way of persuading exim _not_ to attempt delivery of other
recipients of those messages being scanned at this point?

In an ideal world it would be possible, by some suitable
'smtp_etrn_command' script, to batch up ETRN requests which had arrived in
quick succession (say within 30 seconds) and launch a single 'exim -R
foo.com bar.com wibble.com' type process. [The same might be achieveable
by an appropriate regexp I suppose, but this would be cleaner
syntactacly]. Is this functionality possible?

This is my first serious exposure to Exim in the flesh, as it were, so
it's possible that my trawling of the docs, faq and list archives has been
incomplete in some way. I crave enlightenment.

John