Lisa, welcome to the forum. Sorry to hear about the failures, but glad to hear that you are persevering!
I'll start the ball rolling on offering some possible answers, and hopefully others will chime in with true words of wisdom.
First off, if you are new to cheese making, you might want to make sure you can get predictable results with the freeze dried cultures first. Kefir is a very dynamic product, which can differ greatly from one person's batch to another's, and even for the same person will vary over time, depending on which bacteria and yeasts have dominated. This means that you are working with an unpredictable variable, and it may be hard to nail down where the problem lies. some of the experienced cheese makers here have found that using kefir makes it hard to get desired results even when all the other variables are controlled.
For the rennet, what type were you using before? You mention getting walcoren tablets - I was not familiar with these, but when I looked them up they appear to be dry calf rennet. This should be good; if you were using junket rennet before, you should see a definite difference. Make sure you dissolve them in cool or slightly warm
chlorine-free water, and don't prepare it too far in advance before using - there is a definite time limit here before the rennet begins to lose its effectiveness.
Since you are using pasteurized milk - which is true for many of us - you will need to experiment to find a brand that works best. Ultra-pasteurized milk simply won't form a curd; the high heat has done too much damage. If the label just says "pasteurized" rather than "ultra-pasteurized," there appears to be some variation in how high a temp is used - some brands just don't work well, or not consistently. If you can find low-temperature pasteurized (sometimes called vat pasteurized), grab it - that will be your best bet, especially if it is also non-homogenized. (Homogenization does nearly as much damage as heat.)
Around here, my only reasonably priced and accessible option is pasteurized and homogenized; I found to my surprise that the cheapest brand in the supermarket works about as well as anything; at least it lets me form a curd that gives a clean break, and it can make some very good cheese. However, based on my one experience with raw milk, I would say that no pasteurized milk will ever give you anything close to the firm curds and extra depth of flavor that raw milk will produce.
Yes to the calcium chloride - the processing and cold storage of the milk makes the calcium less accessible, and the kefir won't make any difference to that aspect.
Do you have a pH meter? For many cheeses - mozarella and cheddar being two in particular - hitting the right pH markers is critical for getting successful results. From what I've read here, kefir can make it more challenging, since it adds acidity of its own.
Final comment: if you were doing well with mozzarella prior to switching to kefir, you should definitely persevere - most of us have concluded that moz is the hardest of all cheeses to make!
