Scenario1 could possibly end in a tie with gaara being forced to use sand shell.
The big issue with this match is "speed " not just any speed but the speed of movement , does neji have the speed needed to outpace gaara's auto sand , is neji as fast in movement as weighted lee who could avoid gaara's sand , even capable of gettin up in his face on numerous occasions only getting impeded due to lacking sufficient "striking " speed to compliment his movement speed in weighted form.
For us to find out if neji has the movement speed of unwieghted we have to take a look at his feats , his data book stats which ranks him at the same speed of base lee and the fact that his fighting style does not require him to boost his speed much as goken users like Lee.
Some of his agility/speed feats includes him getting sangwedged by naruto's tajuu bunshins and was able to dance around them fluidly without ggettin grabbed or touched while the likes of sasuke w/chidori got tagged with ease , tho he countered naruto he still got tagged.
His other agility feats includes him avoiding rainim amounts of kunai coming from all possible directions , dodging each and everyone at ppint-blank range which gives him more than enough reaction and body speed to easily avoid all gaara's sand attacks if his byakugan can see them .
Now if gaara manages to catch neji somehow , then he can simply expel his chakra to repel all the sand without even needing to rotate so yes neji has a perfect defense and its not rotation.
So with neji having the agility to avoid gaara's sand and get up in his face like weighted Lee did then all that matters is if his attacking speed will aid him unlike lee's did , its also possible that neji can unleash a substantial amount of jyuken in his attacks to meet the chakra in gaara's sand when his hands connect , which should or shouldn't bust through the sand in a similar fashion to wat neji did to kisames water-prison.
If it works then gaara gets injured then proceeds to use sand shell.
In scenaro2 neji will have a much easier time against this gaara since he lacks auto-defense and conventional sand attacks and relies on brute strength and cqc tactics to fight which is why naruto stomped him with taijutsu in that form.
Gaara in that form does not have speed even close to unwiegthed Lee based on feats meaning he won't simply blitz neji and kaiten easily knocked him away followed by a 64 palms while gaara is knocked off balance.
Neji wins scene2 mid diff , naruto manga is all about match-ups and turning gaara into a cqc brute ain't helping him to beat neji more-so than his standard form.