Yes; aside from its noun meaning, HELMET is a verb meaning "to cover or protect with a helmet." And the perfect past tense of this word is HELMETED.
Helmeted Honeyeater was created in 1867.
Helmeted Guineafowl was created in 1758.
Helmeted Hornbill was created in 1781.
No. Hiv is not allowed in Scrabble.
No. Ni is not allowed in Scrabble.
No. Suq is not allowed in Scrabble.
Yes. Jibs is allowed in Scrabble.
No. Abbreviations are not allowed in Scrabble. DIY = Do It Yourself
Acronyms are not allowed in Scrabble, but nairu is an allowable Scrabble word.
No. Tn is not allowed in Scrabble. If you are thinking of TN as an abbreviation, abbreviations are not allowed in Scrabble.
Up your bum
Yes. Any word that meets the Scrabble rules is allowed (e.g. allowed and aloud both are acceptable Scrabble words).