People make a big deal out of MIM. Colt's been using outsourced MIM parts for decades and nobody complains (hardly). Don't knock MIM, Its better than IC when it comes to small parts with fine finishes and surface textures. Think of MIM as evolved IC. IC works well on big parts that will be final machined at a later stage. It can come up with a 80percent part very quickly and inexpensively. If Ruger can IC a Mini-14 they can do any Marlin design they desire.
Forget "what machines and tooling they got". That is of bare concern to Ruger I'm almost certain. Oh, some they'll want, but I would not be shocked if some of it went on the surplus market before long. A multi axis CNC machine by Fadal... whoever... can do any task its programmed for, and has the tooling made for. If machines are decent, tight and current... Ruger will keep 'em. If they bought a load of old Bridgeports and such, and we don't know what Marlin was using, then expect Ruger to do it their way.
In manufacturing, you never want a machine to have down time. You keep it running making part X today, and part M3 tomorrow, and part 2AF221 the next day. The machine doesn't care. Maybe not a daily basis, but you run a batch of parts and the tooling is made by hand to fit that machine to make that part - over and over again. Manufacturing doesn't make all the parts all the time. They make runs of parts to fulfill foreseen needs. If they run out... well... it might be a wait. Things get a little looser during actual production. They say, we got the mache all set to run 400 parts a shift. That's 1000+parts per day. They might say, we only need 500 parts, but run the machine for two or three days "just because" the setup to run those parts took a while. With metallic parts, its not nearly as bad as with plastic parts that are molded... my direct experience in the injection molding trades... oh boy the stories...
At any rate, Ruger got the name, the trademark if you will, and the designs. What they do from there with production, hard assets, stock of already made parts... a total crapshoot to us, because we have no information on which to make a really valid probable course of action. Only educated "if this.... then that" sort of scenarios.