Sardinia was being coy, because they had to be coy - their position was delicate and required careful diplomacy. But despite his occasional flirtations with Paris and Madrid, I doubt that Charles Emmanuel ever seriously considered siding with the Bourbons, because it would have created an intolerable strategic position in which Spain gained total dominion over Italy, leaving Piedmont unable to benefit from holding the balance between Spain and Austria in Italy.
Just as important is that while France was always willing to make concessions in Italy to win over Sardinia, Spain initially refused to even consider it. Cardinal Fleury proposed early on that Sardinia cede Savoy to France in exchange for part of the Milanese, while Spain would take the rest, a proposal which was not received well in Turin. But it scarcely mattered, because the attitude of Philip and Elisabeth Farnese at the start of the war was that Sardinia could either meekly accept their invasion of Austrian Italy, or get flattened: they’d get nothing and like it (“I will not make war for the King of Sardinia,” as King Philip said). As Elisabeth hoped to acquired the Milanese for Don Philip (along with Parma), she absolutely did not want to hand it over to the Sardinians, and the Spanish “evolved” on this position only after Sardinia joined the Austrian forces and continued setbacks demonstrated to the Spanish that wooing Sardinia might be advisable (although the French always remained more committed to securing Sardinia’s cooperation than the Spanish).
Sardinia was offered some pretty sweet deals by the Bourbons later in the war, particularly when d’Argenson was negotiating for the French, but nevertheless was not tempted. One proposal was that Sardinia (the island) be ceded to Don Philip while Charles Emmanuel would gain the Milanese and keep his royal rank by becoming “King of Lombardy;” another proposal by Louis XV involved the cession of Savoy and Nice to Don Philip, with Charles Emmanuel presumably getting most or all of the Milanese in return. But the Sardinians suspected, for good reason, that the Spanish would not abide by these French promises, and they feigned interest only long enough for Austrian troops to return from Silesia in 1746, whereupon Sardinia immediately rejoined the war against the Bourbons and drove them out of Italy.
The only way I can see to get Sardinia into a neutral stance from the start is to have the Austrians collapse so quickly and utterly in 1741 that standing with them in Italy appears suicidal. Charles Emmanuel is not stupid and will not side with an obvious loser. But if this happens, then Spain has no reason to offer the Milanese (or anything else) to Sardinia, so they get nothing out of the war.
As for Britain, siding with the Bourbons seems quite impossible to me. Britain and Spain were already at war, and both the British and French expected that their countries would be at war any day now, not exactly a conducive situation for the forging of an alliance against Austria which Britain really had no reason to entertain. The British people, and Parliament, already suspected George of favoring Hanover over Britain (which was arguably true); if he proposed to betray Britain’s continental ally Austria and side with the Bourbons in exchange for some additions to Hanover, heads would explode around the kingdom. Realistically, the most one could hope for with Britain is for Walpole to remain in power longer, avoiding Carteret’s more belligerent tenure, but even Walpole’s government voted to give generous subsidies to Maria Theresa.
I can see two possible ways to achieve a Bavarian victory in the war. Option one is a complete Austrian collapse in 1741-42 in which the Nymphenburg terms are carried out (or something like them); this probably requires either some really idiotic blundering on the part of Austrian commanders (that is, much more idiotic than usual), or some sort of attitude adjustment on the part of Maria Theresa or Frederick. I doubt much can be expected of the Bavarians, who were rather shit and little more than French auxiliaries anyway. Option two is averting Max’s defection and peace with Austria in 1745, which requires either the slightly later death of Charles Albert (a few more months might do it) or swifter action by the French, who were stymied by Louis XV getting smallpox and Noailles deciding to besiege Freiburg im Breisgau instead of pursuing the retreating Austrians for some dumb reason. If the French move promptly to occupy Bavaria with a large force in early 1745, Pfaffenhofen is averted, Max may be convinced to maintain his father’s claim to the empire, and since France’s original war plans for 1745 were to prioritize Germany over all other fronts Maria Theresa is going to have her hands full trying to fight both Louis and Frederick at the same time.