What an interesting question!
Note: I will be using a dash to separate parts of the sentence for clarity. Technically, these sentences do not strictly require it, and its usage is considered to add emphasis or contrast to the juxtaposition.
First of all, I want to point out that you have made some mistakes with your examples. I don't want to simply edit your question because I feel like I must let you know about that explicitly.
While Russian is much more lax with its word order than, say, English, it can only do so with parts of speech that can show their dependencies using other tools, like changing their form. In your examples, both ни and бы are out of place—since they can't change their form, they have to stay by the words they are attached to:
Что бы ни я делал... -> Что бы я ни делал... (otherwise it would seem like ни is referring to я)
Куда бы ни я ходил... -> Куда бы я ни ходил...
Сколько раз я бы ни пробовал... -> Сколько бы раз я ни пробовал... (this is less confusing, but you still shouldn't do it; a good analogy is splitting infinitive in English—you technically could still convey your meaning perfectly, but it is better to avoid doing this)
Secondly, when it comes to translating these sentences, you should look to the second part of the sentence for clue with selecting the correct time.
For example, both of these phrases are correct and are referring to the current state of affairs:
Both of these phrases translate to:
Whatever I do—it all goes south.
I would say that the first variant is a bit more blunt. It has a stronger message because it implies that it is not just things that you have already tried in the past that did not work out—it is instead something that happens even now, so your misfortune did not stay in the past and is an ongoing issue.
However, to my knowledge, there is no way in English to both maintain the general present tense of the sentence and to show this kind of distinction.
However, if we are talking about events in the past, the sentence would be:
and it translates to
Whatever I would do—it all would go south.
Here we are using would's ability to form habitual/repeated actions in the context of past events.
But we can also use this construct to talk about things in the future!
Что бы я ни делал — всё выйдет из рук вон плохо.
which translates to
Whatever I do—it all will go south.
You might wonder where did the alternative go on this one. Well... it is technically a correct phrase: Что я ни буду делать — всё выйдет из рук вон плохо.. However, it is so clunky that its use would be situational at best, and most speakers would probably just rephrase the entire sentence rather than use it.
So, as you can see, the phrase can be used with all tenses, not just the present one.
But that is not all! You should also consider the perfective aspect of the Russian verb inside that construct:
Что бы я ни делал vs Что бы я ни сделал
There is no direct correlation between the perfective aspect of Russian and the perfect aspect of English, but in some cases, the translation will be perfect rather than simple:
Чтобы я ни сделал — это всё в прошлом.
which translates to
Whatever have I done—it is all in the past.
Again, I must emphasize that it does not mean that all perfective verbs should be translated into perfect tense: Что бы я ни сделал — она будет недовольна. will translate into simple Whatever I do—she will be unhappy.
Now, onto your question about imperatives. It is indeed an alternative for the second person. So the following are correct:
Present:
Что ты ни делаешь — всё выходит из рук вон плохо.
Что бы ты ни делал — всё выходит из рук вон плохо.
Что ты ни делай — всё выходит из рук вон плохо.
Что ты ни делал — всё выходило из рук вон плохо.
Что бы ты ни делал — всё выходило из рук вон плохо.
Что ты ни делай — всё выходило из рук вон плохо.
Что бы ты ни делал — всё выйдет из рук вон плохо.
Что ты ни делай — всё выйдет из рук вон плохо.
And, of course, you can drop that pronoun for brevity if it is obvious from context (or from your use of imperative) that the subject is indeed the second person.
As for your second question: the direct translation of No matter how good you are/However good you are would be
Каким бы хорошим ты ни был
That being said, a short form is begging to be used there:
Как бы хорош ты ни был
I should point out that full form of хороший implies goodness of heart, or of material, etc. But for your example, I get the sense that the phrase means more of a "master", which is better reflected by the short form in Russian. That is specifically for хороший, with осторожный you could use either:
I'll also point out that in English, "how" has the connotation of both quality and quantity (as in "How much more careful can I be?"), so in Russian, it could also be appropriate to translate that phrase as:
It gives it that same feeling of the quantitative assertion.
Finally, I want to also cover an interesting edge case.
These phrases are correct and absolutely can be used:
Что я ни делал — всё выходит из рук вон плохо.
Что я ни делал — всё выйдет из рук вон плохо.
Что я ни делаю — всё выйдет из рук вон плохо.
I would say that the difference in time between the parts of the sentence is used to specify that described actions are lengthy and have a somewhat delayed nature.
So in the example, it definitely will go south, but there is a delay between me doing something and it turning bad.
