Hey Jude, don't be sad...
Take a brown stock and make it better...
Remember to not overthink this task...
Then you can start to make it better.
Da da da da da
da da da da
That's my lame attempt at getting you to relax about the stock making process. If only everyone was as enthusiastic as you are with your practicing.
#1 Usually the remouillage and initial stock are not combined as the remouillage - even though you say it tastes good - will weaken the color of the initial stock. Usually, a remouillage is used to add to stews or soups (rather than just plain water). If combining the two works for you, that's all that counts.
Don't over think your stock so much. Yes, skimming will make the stock clearer...but if you didn't get the chance to do it initially, don't worry about it. What is important is that you taste the stock and if you are happy with the results, then work to find a balance that works with you. Many kitchens start veal stock at night and leave it over night to simmer...and there is no one there to constantly skim and fuss over the stock.
#2 The smaller the bones, the easier it will be to extract the gelatin...but there is no need to get carried away. Obviously if the second stock was gelatinous, the bones were small enough. There will always be some matter in the marrow. It won't completely dissolve.
#3 - 14 hours is a long time. There is a point where you can over do it and the mirepoix will start to disintegrate and cloud the stock. Raise the temperature so there is slight activity going on in the pot. This was discussed in
this thread. Get out of the habit of using a thermometer to make stock and trust yourself by looking at the indicators. You might want to review the lesson on
Submersion Cooking Methods, Topic 5 (Simmering). You are making some good observations, but again, don't over think it.
#4 It's hard to say what the brown thick stuff was. It could be impurities stuck on the bottom of the pot, which is why it is best to not stir stock while it's cooking and to gently remove the bones and mirepoix before straining. Experiment and see. Strain some with a strainer or cheesecloth and strain some with the cloth that you are using. See if you can notice a difference in the flavor, color and clarity. Don't press solids through the strainer - this can add bits of mirepoix which will cloud your stock.
Last question - Interesting set up that you have rigged up to keep a constant temperature. Review the simmering topic. The bottom of the pot will always be hotter as it is in direct contact with the heat source. The main thing is to just make sure your stock isn't BOILING! A light simmer is ok. Don't worry so much about it.
My one bit of advice is to make smaller batches. I know for veal stock this is probably not worth it, but a white or dark chicken stock is just as good (if not better...at least we often do it here). We don't want to scare people away from making stock because it doesn't have to be such an ordeal. A couple of leftover carcasses are enough to get a small pot of stock on on a regular basis...and it's literally pain free. We also find that with smaller batches sometimes you don't even need to freeze them. Just keep them in the refrigerator and use it fresh as you need it throughout the week.
Good that you are tasting things. You'll begin to understand what to and what not to incorporate into your cooking.
Okay, after all that, I hope you also aren't having "stock" nightmares. Put your feet up and get some rest. You are cut off on the stock questions for a while (just kidding)...but really, don't over think it so much. You are leaps and bounds ahead of most cooks out there by just making your own stock. Cheers!