Are these examples of using commas inappropriately because they separate subject & verb?

1) "The hesitant student comes up first, releasing a howl reminiscent of a wounded animal."

2) "...until a dust cloud appears on the horizon, approaching fast."

Also with sentence #2...should "quickly" be used instead of "fast" or should it be "fast approaching" or is it okay the way it is? Thanks.
They are fine; they do not separate the subject (student, cloud) from its verb (comes, appears); they merely offset the nonfinite clauses.

I would use quickly.
Hello TaraC

I think you need the comma in your sentences. There the present participle construct is used to give additional information about the subject, and the rule is to put a comma in such a case. Otherwise, some confusion could take place. For example, let's compare the two sentences below.
[1] Bill gazed at Monica, talking on the phone.
[2] Bill gazed at Monica talking on the phone.
In #1, the person who was talking on the phone was Bill, but in #2 it was Monica.

As to the modifier for 'approach', I have learned, in general, 'quickly' is used to state about an instantaneous action, whereas 'fast' is used to talk about the speed of a durative activity. So my choice is rather 'fast', but I might be wrong.