Yes, you can add mal- to any word to form its opposite :)
And mostly that's what you do - there are some pairs of opposites where both separate words are in common use, but often, one of the opposites is only poetic or is not there at all and you use mal- to form the opposte, as with malfermi "OPP-close = open", malgranda "OPP-big = small", maljuna "OPP-young = old", malsana "OPP-healthy = ill", maldekstra "OPP-right = left", and so on.
Sometimes, people even add mal- as a joke when it isn't usually used, e.g. maltrinki "OPP-drink" for "piss" or malmanĝi "OPP-eat" for "throw up".
With regards to your specific question -- it only applies to some words. Some words have no opposite (malarbo, malsegxo, malpomo), some words don't have a clear opposite (malknabo, malbrui). Other words have a clear opposite but are expressed by separate words (blanka/❤❤❤❤❤, nordo/sudo).
Neverthless, mal- is very useful and can be used to make any opposite, as long as the opposite is clear.
Translations of examples above - untree, unchair, unapple, unboy, unmake a ruckus, black/white, north/south.
In the meaning "have an opinion", "think" is not usually used in the present continuous tense.
You can say "I am thinking about it" if you want to emphasise the cogitation and mental work behind it, but for "I assume, I am of the opinion", it's usually "I think (that)..." - much as we say "I see that house" and "I have a pencil" rather than "I am seeing that house, I am having a pencil".
It sounds good to me.
There are three things that happen when we accent a syllable.
- we say it louder
- we hold it for a longer duration
- we use a higher pitch
To my ear (fluent Esperanto speaker) it sounds like he is correctly accenting the syllable, although there seems to be equal or higher pitch on "vas" - which might be what your ear is picking up on. Two out of three ain't bad.