HELP! Forestay Crisis

[Brion Toss]

Hello,
Fist, halyards will likely be strong enough to keep your mast forward, especially if assisted by the forward lowers, but it isn't a setup I would ever count on. For one thing the vector of the halyard moves its effective pull well aft. For another there is a lot of elasiticity in the rope. And for a third it just isn't as reassuringly reliable as actual standing rigging. So you might start by easing off on the backstay a bit, then putting a sling around the mast at deck, and hauling it aloft with the main halyard, with a strong, low-stretch piece of rope or wire led forward from the sling. Once the sling reaches the spreaders, lead the line forward and take up with a handy-billy or come-along. Next, put Icicle hitches on the bottom of your forestay (inner stay) if it is, as I understand it, made off aloft. Take the stay forward and tension it with a purchase as well.
The oscillations might be in part due to vortex shedding, and I believe you can find fix-it's for that problem in the archives.
Since to my knowledge you are in an area that is scant on qualified riggers, you are probably on your own there. So once you get the rig a bit more reassuringly stabilized, take a breath and be in touch again. The level of work you need to do is well outside the format of this board, so please consider at least setting up a consultation with me or some other qualified rigger on how to proceed with the rest of the rigging. As you have discovered, owning a dozen boats doesn't make you a rigger, any more than rigging a dozen boats makes you an owner.
Fair leads,
Brion Toss