Permanent magnet return is a good idea. The force generated will usually be highly non linear. As the magnet attracts a pole piece, the air gap will steadily reduce and the attractive force will greatly increase.  Likewise as it moves further away, the attractive force will diminish rapidly.
The effect would be the return magnet would tend to "stick" and then fly apart. A coil spring will offer a much more linear force versus displacement characteristic.
The only solenoids I have seen that use permanent magnets are latching bistable relays, they snap from one position to the other, and then hold (latch) magnetically, without requiring any electrical energisation. That is the least desirable sort of characteristic for the linear positioning of a solenoid.
Commercial solenoid valves will have a valve body, with threaded pressure fittings, just as you suggest. These usually dismantle into at least two major sections.  It is possible to fit just the solenoid and actuator part directly to your own manifold, provided your manifold is machined to duplicate the original valve seat. That can  eliminate much external pipework, and greatly reduce gas volumes and the possibility of leaks.
I have seen this done in medical equipment, where a machined manifold has solenoid bodies attached directly to a common aluminium manifold block.  It makes for a very simple and compact assembly. Other fittings such as needle valves and pressure transducers thread directly into the same solid manifold block.

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