sv8100 inmail question

No. Better to use a 77+ext number on a key to get to the vmails.

No, even if you forward calls to 301 to 321, the only way they well end up in the 321 voicemail is if somebody physically picks up the phone, and transfer it over. Best solution is just a function key to access 301's voicemail.
One client of mine REALLY wanted their entire office to share a voicemail. So I gave them a convuluted solution with virtual loopbacks, then everybody the phones busy n/a to a DID entry that went directly the one voicemail. Obvious drawback of doing that is that you ruin the caller id, and it's convuluded. At least, this worked on a sl2100, but NEC support told me that loopbacks were only supported on the older systems, deprecated on the newer systems, and they actually force me to unprogram my loopbacks before they will provide me any support........

Put an inskin voicemail for 301 on 321 maybe

Yes, there are two ways to do this, first you can use the DID table to send incoming calls for 301 to 321 and as this is applied at this level, the calls are considered by the system as having an initial destination of 321 so calls will end up at the 321 mailbox. Second method I would have to check up on but IIRC there is a way of putting a code in the DID table that effectively sends it to the mailbox you specify there, but I have never used it myself and would have to check the documentation in order to help with that. Maybe Belvedere can help with this method.

It would all depend on what type of lines you have , Copper,PRI or SIP?