Hi John - Cables can be a controversial subject but, in my experience, the RCA adaptor which Audioquest supply works well and I imagine there are others which will be equally good. I recommend doing that but how much difference it makes to your specific set up I cannot say. I can say that the difference a decent DAC makes over proprietary hardware can be huge and you will not want to go back. You may want to upgrade to an AQ Red but, as a first step, this seems a good place to begin.
There are others here who can give you a much better response to your second question but, in a nutshell, Audirvana had Direct Mode as one of the set up options which works like a bypass to the MacOS core audio elements and for some it remains the optimum arrangement for those using an Apple device. I find it is essential whereas others say it makes no significant difference; hard to understand but each to their own. The reason I brought it up is because you are running a 2010 MacBook which tops out at MacOS 10.13.6 (which is what I have on my MINI). If you had a device which could run a later version of the MacOS your options to use Direct Mode may be constrained, though I have not heard much on this subject lately. In earlier versions of the MacOS no patch was required. The problem arose through (unreasonable) changes made by Apple which we all just have to work around.
You will probably find that running Audirvana in its standard set up will be very rewarding and, when you have become used to it, you could try doing the kext patch to allow Direct Mode and see what you think.