I still dont understand ororchimaru's arms.

[Honord Sage]

Edo and other for bitten jutsus were taken away from Orochimaru and sealed away, not all and the body Orochimaru had at that time lost control over the use of His arms. When Orochimaru change bodies He regain the use of His hands but not the abilities of forbidden jutsus like Edo that had ben sealed away now by using the mask He has regain all the jutsus He lost.

 

[Ero sennin jiraiya]

We've seen others use hand seals to do summoning jutsu but that doesn't mean it's a requirement for every type of summon. When Orochimaru is seen using various summons like the rashomon gates, all he does is bite his thumbs and touch the ground. It's possible he just doesn't need seals for summons. In the case of the snakes, his tattoo might have something to do with this. Most other ninja who have a summon don't have a summoning tattoo as it isn't a requirement.