Has anyone here ever tried the lessons? I think some of them may need some corrections and updates.  Let me know your thoughts.

The Swedish LARP-orcish Svartiska was not really created by a single LARP-group but by the community of orc-larpers where different groups created different dialects.

On pronunciation

There is an orc name that begins with y - Yagul - in The War of the Ring (The History of Middle Earth, vol. 8 )

I think Tolkien pronounce Mordor in Elvish, it is after all an Elvish name meaning Black land in Sindarin (or "shadows" in Quenya). It has nothing to do with pronunciation of the Black Speech.

And what about the sounds in the excercise: -qu- in "throqu-" and sr- in "srinkh-"? Especially -qu- seems out of place. Why not spell it kv or kw?

Yes, there as some issues with qu, specially when next letter is also u. Could be also spelled like Q. It appears only in words borrowed from elvish languages. I will replace it with something else if I would create new dialect.

I think there is nothing special with sr, for me it's easier to say than thr (thrakatulat).

Does comparative and superlative adjectives, and adverbs mark plural?

The dark tower - lugbûrz; the darkest tower - lugbûrzaz; the darkest towers - lugbûrzazu

urukû ghâshuzat hîzarz lug "the old orc quickly burned the tower"; urukûz ghâshuzut hîzarzu lug "the old orcs quickly burned the tower"
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edit 1. I saw that the adverb is not agreeing in number so: urukfuz ghâshuzut hîzarz lug
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edit 2. I saw that I somehow confused the adjectives - this i now corrected.

I think adverbs do not have plural form. Adjectives do in any form

There are two collective plural, -hai and -ûk. In contrast to the ordinary plural these can be used with people and races. So we have uruk-hai (the orc people) as the most famous example. And then in the lessons (IV) there is an example of the -ûk ending used with  sharkû (old man) > sharkûk "all old men". So both the collective plurals can be used with people and races but what is the difference between them. What does sharkû-hai mean "all the old people" or maybe "the society of old men" or is it equivalent to sharkûk? Or is it just gibberish.

In lesson XIII on suffix order, verbs collective #6 two endings are given, -ûk and -âzh. The -âzh ending is used with a verb 'ufubulâzh' (will frighten them slightly). I cannot find this -âzh in the lessons or in the wordlists (there is "azh (conj, HORN) "also").

It seems to mean "slightly" but then it is not a collective. Confusing

I think it's Scatha's mistake.
I don't like interpretation of -hai as collective plural suffix nor simply as "folk", "people of" etc. However I can't offer better one.

I've added this shortly before my HDD crashed. Online version of dictionary is not updated still.
Here -uuk and -aazh are something like verb's aspect (perfect and "partial" respectively). Interpretation of "-uuk" as "completely", "fully" is taken from A. Nemirovsky's analysis