Have you done any more tests to see if the Remington caps solve the problem?
I don't know if there is a "direct" 209 conversion for the Marlin. There's a possibility that the TC Black Diamond musket cap nipple will work.
But when it comes to converting to the 209 I don't know if the available TC conversion units will fit or work or not.
The Marlin has an ambidextrous bolt and I don't think that any of the other TC rifles do or if that matters.
A new striker may be needed.
There's 2 conversion kits for the Firehawk and Thunderhawk and System 1. Coincidently the scope base for the Marlin is the same as the Firehawk and Thunderhawk, but that doesn't mean that the bolt would fit.
Also, the Black Diamond has a 3 nipple system but I don't know if they all use the same striker.
I would try the Remington caps and try to adjust the taper of the #11 nipple if necessary.
Then I would try the musket nipple next.
Only if those attempts don't work would I try to install a 209 conversion. That is unless I were more certain that it would fit.
Musket nipple:
https://secure.tcarms.com/store/flam...t-no-7237.html
209 Primer adaptor:
http://www.midwayusa.com/viewProduct...tNumber=686477
Conversion units for TC guns:
https://secure.tcarms.com/store/flam...t-no-7776.html
http://www.midwayusa.com/viewproduct...tnumber=320263
http://www.midwayusa.com/viewProduct...tNumber=817982