"If I was IGO, I would buy 19.99% PAN. Then, when my bid for WSA...

"If I was IGO, I would buy 19.99% PAN. Then, when my bid for WSA is successful, I will own 40% PAN, which I tried to buy outright some time back at higher sp. Could end up owning both WSA and PAN."

That's a cunning plan, but unfortunately not quite cunning enough. Notwithstanding the various exceptions to the rule, which would almost certainly not apply in that situation, the Corps Act prohibits holding more than 20%, regardless of how it is done. Not even using the method described above. IGO would need to lob a formal bid for PAN first. And they just might down the track, if they're successful with their WSA bid.

"If I was BHP. I will buy POS shares up till 4.99%. Then, I will go full force to buy the maximum shares I can within the time limit I have before I am legally obliged to disclose being a substantial holder. Then, I can even do a deal with BMM traitors who absolutely look like they are willing to sell."

Maybe, but you need to understand that the ongoing disclosure obligations for substantial holders is based on percentage holding movements (i.e. movements in holdings of 1% or more), rather than any time-based limit. Disclosures are not expected to be done in real-time and there is a time lag for reporting, which will have its own limit (think ~1 week, max). The point here is that disclosures would be required in a timely fashion every time the holding increased by 1% or more. Each 1% change in holding equates to a minimum change (addition) of 30m shares for POS. The element of surprise which you alluded to would be long-lost soon after the F603 (initial substantial holding notice) landed and most definitely by the time the first F604 (change in substantial holding) was disclosed.

In order for that plan to work (from a 4.99% starting point), any such player would need to acquire a (maximum) 460m shares (15%) in a very short time frame (i.e. less than a week), in order to quickly take their holding from 4.99% to ~20% before they needed to submit their initial F603. It's an interesting theoretical scenario, but completely impractical via open market ops, given our daily trading volumes. Approaching an existing larger holder (or holders) for a single off-market purchase (or a couple of concurrent off-market purchases, in the case of securing agreement from several large holders) would be a more likely pathway in executing a surprise raid. The more holders required to line-up the more complex and difficult it would become. An unlikely outcome without securing an off-market sale from at least AF or BMM for their respective ~12% POS holdings.

On BHP:
If BHP was concerned about protecting their dominance in WA Ni, then (imo) they would/should lob a competing bid for WSA. Go hard and pay a premium on the premium (i.e. above the current premium already on offer) to strategically stymie IGO's aspirations. BHP successfully bidding for WSA would:
1) Add more strategic Ni units to their already-dominant WA portfolio, including a strategic 20% holding in PAN which would lock any possible PAN suitors out; and
2) Undercut IGO's ongoing Ni player status. Losing the WSA bid would cut them deeply, given the limited remaining lifespan of Nova and not much else to replace it at this stage. This could potentially (eventually) weaken IGO to the point of exiting the Ni space completely and becoming a pure Li play when Nova reaches EOL. Naturally all that depends on what happens with IGO's other Ni exploration efforts (e.g: Fraser Ranges, which has so far failed to deliver a Nova replacement).

Regardless of whomever eventually successfully gets WSA (and, by extension, the strategic holding in PAN), it would leave MCR and POS as the next players in line, of which AF holds 15% and 12%, respectively. Not enough to block a SOI T/O in his own right (it would require 25%), but more than enough to command a voice at the table. And enough to block a hostile on-/off-market T/O bid (i.e. non-SOI) in his own right (requires only 10%). Same goes for BMM's of POS. They're much more likely a seller for the right price, but enough to make any real difference in the outcome?

On IGO:
IGO has the first-mover advantage in much-anticipated WA Ni consolidation game, but it could vapourise in a heartbeat if a compelling WSA counter-bid was lodged and ultimately successful. And being the junior holder in MCR (holding 'only' 8%, behind AF's 15%), any consolation play to acquire MCR would look shaky from the get-go.

On POS:
As for POS, I don't see BHP bidding. WSA is a more compelling target for them. However, even if they did bid, I think they have two chances success (Buckleys and none), given AF's material presence on the register. They're smart enough to learn from the recent Noront experience which would/should give them an good idea of their chances re- POS.

There's also the very recent precedent for AF offering a "remain inside the tent" (and continue to hold) option for existing SHs of a target, per his Noront offer. So, if he's a player (almost certainly) in any bids (by any others) that come our way in the medium-term, then I wouldn't be too quick to write-off the possible opportunity for current holders who wish to continue to ride the thematic in POS to do so. But only because our interests might align with AF's on a given day and certainly not because of any sense of corporate benevolence/philanthropy.