Calculate total volume group capacity from combined physical volume extents.
A Volume Group pools the Physical Extents from every Physical Volume added to it (vgextend), so its total capacity is simply the sum of all member PVs' usable extents — this is what lets LVM present multiple physical disks as one flexible storage pool. Free PEs (total minus those already allocated to Logical Volumes) determine how much new LV space is available via lvcreate or how much an existing LV can grow via lvextend.
VG capacity
VG_capacity = Σ(PV_usable_extents) × PE_size
You can't — all PVs in a single Volume Group must share the same PE size, since VG capacity is tracked in units of extents. If you need a different PE size, it must be set consistently for the whole VG at creation time (vgcreate -s).
Yes, if there's enough free space elsewhere in the VG — first run pvmove to relocate any extents in use on that PV to other PVs in the group, then vgreduce to remove it, and finally pvremove to release it entirely.
Add a new physical disk or partition, initialize it with pvcreate, then run vgextend <vgname> <new-pv> to fold its extents into the existing volume group's free pool, immediately available for new or growing logical volumes.