There are a lot of issues here, it will be quite the wall of text...
Using -tir as "guard(ian)" is debatable, though good enough. But "glanir" is definitely not how it would be compounded with glan. A late compound would be glandir (which can also be viewed as "border-man"), but since you are not talking about the guards and rather the swordsmen (etc.) of the guards, you should rather use glandirith (using tirith, the act of watching, guarding).
Genitive formations are implied with word order (cf. Aran Moria = "King of Moria"), not with the preposition o which actually means "from" and does not make sense if the soldiers are still in Lórien. There is a word for "of the" (not just "of" though), but its usage depends on the following word, rather needlessly complex for what you need, since "the" is not always required. So, just use Magor Glandirith Lórien "Swordsman of (the) Border-watch of Lórien" = "Lórien Border-watch Swordsman". And you only need to pluralise magor in that formation.
Next issue is "îthron". Sindarin does not allow overlong vowels (i.e. the ones with circumflexes like "î") in polysyllabic words (with very specific exceptions). Only long vowels are allowed ("í"), and not even them in the last syllable (except old names) or if they precede a consonant cluster like "thr" which is the case here. But no, the plural of eithron is not "ithryn", because "ei" does not change when pluralised. So its plural should be eithryn, though I would personally use eithor & eithyr.
I was asked about the singular form of Galadhrim not long ago, and I offered (m.) galadhron & (f.) galadhril. I guess, the word got around. However, it is not what you should use there. It would be the same as in English, only reverted. Thus Maethor Galadhrim "Warrior of (the) Treefolk". You can use galadh[ron/ril] only by itself (without "warrior" etc.) if you want to address a single Elf of the Galadhrim.
And now "tir" is not even a suffix. This is even more unsafe than the using the suffixed form as "guard(ian)". I would suggest tirron & tirryn. And again, drop the o "from".
A female minstrel would be pethril.
I would replace Taur e-Ndaedelos with Taur-nu-Fuin. Because Tolkien wrote the latter later (so it is more of "final decision" than the other), and because the word [n]daedelos is slightly problematic, long story. Also the forest was not always called "Mirkwood". Before the influence of Dol Guldur, it was Eryn Galen "Greendwood", and after the War of the Ring, it was Eryn Lasgalen "Greenwood the Great (lit. Forest of Green-leaves)".
A "palace" would be orbar(dh) like the Quenya word oromar(d-) "lofty hall, high-mansion, high (lofty) dwelling". The initial element shares its root with orod "mountain, hill", similar to the etymology of English "palace" which comes from Ceaser's house on "the Palatine Hill".
Well Avari is a Quenya word, and one should not mix languages. It is Avar & Evair in Sindarin. But you can be sure that no warrior in Mirkwood would be called an Avar, so maybe you should drop this altogether.
You can use glandir by itself here. Its plural is not "glendir" though. What causes Sindarin words to change form when plural is an ancient final "i" which disappears after affecting the words. But here there is already an "i", so the preceding "a" should have become "e" already. Meaning that it would be "glendir" both in singular and plural form. But, there is also a reformation rule with which recognisable elements in compounds are restored, hence glandir, and another "i" would not really change that. So "sg. glandir & pl. glandir", but the difference can be marked with the help of tirron in the form glandirron & glandirryn
The "elk riding" thing is a (rather unrealistic) invention of the Hobbit films, so it is not really a thing unless you want to go for it. The word is phonetically off however with that "...v+b...". And by that "v" I mean: Tolkien used final "-f" in the Latin orthography as "v" instead, so it is actually "celev" and should be written like that when that V is not the final letter. More importantly, "-ben" is not really what you need there. I assume, you were copying rochben "(horse)rider" (because I also saw "wind rider" in your list as "gwaeben" which... you will see...), but this "-ben" does not mean "rider", it is simply the word pen "person" in a compound. So the sense "rider" comes from the fact that "horse+person" can only mean "rider". Meanwhile a "deer+person" would arguably mean "hunter" instead (and "gwaeben" would mean... "?weatherman", basically nonsense). Anyway, celef is a neologism with an existing attested alternative aras, so it would be better to use that. You can use it alongside northor & nerthyr "rider(s)" as northor aras "rider (of) deer = deer rider".
That "ecthelorn" is a neologism whose justification for that non-final "c" (not allowed) does not really work in Sindarin. This would be better as eithorn. While we are at it, "soros" is an early draft word that does not really mean anything in Sindarin, I would replace it with feren. And again, the formation should be "Onod Feren" etc. Though I should say, Ents were not really trees...
The form "ionath" is attested so it is fine, but "ionnath" is also attested, and it is arguably more plausible so you may want to change it.
I guess "hatholir" is like "glanir" where the initial sound of the second element disappeared for no reason, so "hathol+chir", which has other phonetic issues. I would rather offer maechir or megilchir with identical plurals.
Not sure how to approach "lancer", I suppose "spear-rider" will need to suffice. So Eithrochben with plural "-bin".
Loremaster would rather be angollon & engellyn (cf. Q. ingolmo "loremaster", Q. ingolë = S. angol & Q. -mo = S. -(r)on).
Again, not sure how to approach "catapult", maybe hadiod from had- "throw" and gaud "device". The phonetic development makes it rather obscure, but there is not much of a choice. The words like sling with "rope" in it have uncertainties around them, so I would not bother.
I would use Borvaethor as "veteran", since the root of the initial element has other derivatives like "old, endured", thus an etymological match for the English word. There are multiple ways to translate the "Last Alliance", I would translate it as Gwennas Dell.
Not sure what to do with "Glorfindel's Wind Rider", I don't get what it is. I can only say that Sindarin word means "wind-person" instead.
As usual, revert the Dúnedain formations, it should come after the troop word. As for "ranger", I would not use that "stealthy" word, it is a very stretched neologism... Now, it can be argued that the use of "ranger" in the works of Tolkien has two different meanings. When speaking of the "Rangers of the North", it refers to the wandering folk of the Dúnedain as a whole (including women and children). And they were called "rangers" (i.e. "wanderers") by the Men of Bree, so it is doubtful that they would refer to themselves with the same word instead of saying "I am a Dúnadan". Meanwhile the "Rangers of the South" were Gondorian special forces from Ithilien, and here the word more or less meant "soldier", which does not quite apply to the Dúnedain in the north. In other words, you may want to change this.
And finally, poll is the animal "ram", so the final entry is basically "the ram-animal of the Dúnedain", it does not hint at anything else. And it would arguably be an Anglicism to use the word for "ram (animal)" (even with "battering" alongside it) for the siege weapon. So I will instead suggest dringod "beating/hammering-device".
That took a while... But looks like we are done.
