Namaste
Upon going through both the documents. I would like to comment the following ~
1. Most of the ISKCON answers are based on appeal to authority and kind of like beating around the bush.
2. Sri Jagannatha Ji isn't a utsava vigraha but the Mulavigraha himself, quoting vaikhanasa agamas won't work as the Puri temple runs mostly on other tantras neither Vaikhanasa nor pancaratra (except certain rituals). Thus quoting the Vaikhanasa agamas won't prove anything here as it would give rise to tantrasankarta (mixing of both ways of worship) which is condemned in every agama of any genre. Regarding His coming out of the sanctum center, it should be seen as a divine exception given by the Lord himself. The texts followed in Jagannath's worship are the purusottama mahatmya of skanda purana, vamadeva samhita, Niladri mahodaya etc.
3. Sri Vishnusahasranama is from Mahabharata, this can be read by anyone without the nyasa etc. which involves vyahartis or pranava. From the text itself we know "ko etad muchyate jantu", jantu means any bodied living being. Most of the Vaishnavas agree on this, Parashara bhattaji doesn't mention any prohibition as well also modern stalwarts of Madhva sect like Bannaje Govindacarya say that anyone can chant the Vishnusahasranama with proper caution of pronunciation. In the skandapurana the Advijas have been asked to to abhisheka of Radhakrsna with Vishnusahasranam.
4. Through out the ISKCON document couldn't see any quoted injunction of doing the rathayatra at any random date. Jagannatha prescribed 3 dates they may do on any of those dates or during the aniversary of the deity installation as per the pancaratra text they have quoted, but this injunction as well isn't uniformly followed.
5. One may say rules of odissa apply to odissa only, but that is only for rules like eating fish etc. not general dharmik rules which are to be followed regardless of geography. One of them is to do a particular utsava on a particular allocated time per the sastras as the SJTA has shown via the dharmasastras. If all the rules in the skandapurana were to be discarded based on geography, divinity of Jagannath would also face the same fate.