My take on the whole sage goes something like this. At the begining of the war pre Reven entry the Mandalorians has the advantage as while the Republc forces were bigger they were spread out with more territorty to cover. A lot of the total fleet was probably in the core protecting the founding worlds of the Republic. If you add in a bit of complancany with officer promoted for their connections as opposed to ability it explains Mandalorain victories.

Reven shows up and had to take desperate measures to hold the Mandalorians. He does not have total cotrol over all Republic fleets and with the forces he has to work with they are outnumbered hence some worlds being sacraficed in order to save others. Having to make these sort of decisions may well have forced Reven to distance himself from his emotions as how else could he order soldiers perhaps even entire planets to be sacraficed and not crack up.

Eventually Reven & co get the upper hand against the Mandalorian who were being driven back. Reven had already begun to look to the future a bit like some WWII general among the US/UK who were conscious of what the USSR would be up to when the war won.

Reven wants to crush the Mandalorians so that they will not be a treat for a very very long time. He cannot affort to have their fleet scatter into small groups to cause trouble. The Republic forces could spend years hunting down various groups. At the same time Reven wants to consolidate his hold on the fleet and remove the less loyal officers and crews.

So he sets a trap. Send a small fleet to Malacor. It has to be big enough to be a threat but small enough that if the Mandalorians gather all their ships in one place they can outnumber it and possibly slow down if not haunt Republic advances.

I base this on the fact that Canderous said the Republic outnumbered the clans but the Exile seems to suggest that they were outnumbered and had to employ the device to defeat the Mandalorians.

Reven wipes out the bulk of the Mandalorian fleet at one go and the part of the fleet not so loyal to him suffer heavy casualties.

Why was the Exile sent? He may have been not totally committed to Reven's side. Even if he was loyal, the fleet sent there was going to have a real hard fight, they could not just activate the generator straight away. The Exile may have been needed to keep the fleet together long enough to completly trap the Mandalorians and with her ability to form bonds she may have been the only general that could have kept them in fighting form other than Reven and he could not be there.