If I have a port A which has say VPID 200, has VLAN 200 untagged and also VLAN 150 untagged, does that mean that frames on the switch tagged 150 can reach port A, and open a connection, but port A cannot go find IPs/MACs on VLAN 150? I haven't seen this discussed in detail, but does this create 1 way VLANs in a sense?
In principle you can have only one untagged VLAN. This vlan may be called differently based on the brand but all the time it is pointing the same status. You are pointing trunk mode but there is allowed just single one native / default / untagged VLAN.
The configuration should not allow you to set more untagged VLANs and in case you could set it up that way the "last set" wins the selection so my guess is highest VLAN "win competition" about native VLAN ;-).
Available port modes:
You can use one and only this one VLAN on the port and this VLAN is in principle untagged. This mode is used normally for connecting end nodes.
This mode is when there is possible to communicate using more VLANs where one is default / native / untagged (all not tagged traffic is "assigned" to this VLAN). ALL the rest allowed VLANs have to be tagged. Based on the configuration you can allow / limit communication just for listed VLANs or you can allow all known VLANs on the device to go via the trunk port.