While your pictures do look like the mast is pulled back at the upper spreaders, I'm sure you'll find that it is a combination of the lowers holding the mast too far aft and/or the headstay and backstay holding the top of the mast too far forward. (There being no stays exerting fore and aft force at the upper spreaders.)

Re-tune by first totally easing the mizzen backstays so the triactic is slack.  Then slacken the main lowers and the main headstay and backstay. You can leave the intermediates as is.  Next, get the mast in column fore and aft by tightening the head and back stays.  Then tighten the lowers, keeping it in column.  Check side-to-side as well as fore and aft as you tune.  

Before re-tensioning the mizzen backs, loosen the mizzen lowers, get it in column with the backs and finally adjust the lowers.

As a short cut, you might just try to get it back in column by adjusting the lowers, taking up on the forwards and easing the afts.

Cheers,

Craig - SN#68 Sangaris

By "Keys" I think you mean "wrenches" (aka "open end wrenches", or, as the Brits say, "spanners").  There are different sizes for the different stays and, yes, some are odd sizes, seeming neither standard metric nor SAE (inches).  When I can't find the right size, I use an adjustable wrench - and you need a large one for the backstay to get enough leverage.  You can also insert a large screwdriver in the slot of the turnbuckle bodies instead of using two wrenches.  You may want to wrap a cloth on the turnbuckle first to prevent marring the surface.

If you are buying new wrenches, I'd suggest a combination set of both metric and SAE sizes.

Good luck with it - Craig - s/v Sangaris SN#68

After all that work it must be frustrating to have the same problem.  Let me be sure I understand your post. The mast is straight with the shrouds loose.  Then when your tighten the cap shrouds (or uppers) and the intermediates the mast is bending between the lower spreaders and the top, with the upper spreaders being aft of what would be a straight vertical line, that is, it has an inverse or negative bend, which we definitely want to get rid of.

I assume you tensioned the headstay and backstay first to about 15 to 20% of their breaking strength (really, really tight). Then you should be able to remove the negative bend by slacking the aft lowers while tightening the forward lowers and by tensioning the backstay.  You may want to experiment by really slacking the aft lowers and see if you can induce some positive bend by tightening the forward lowers and the backstay - if you can then you can re-adjust to get it straight. 

By the way, there are a lot of good tuning guides on the internet - here's one from Selden Masts that you may want to look at: www dot riggingandsails dot com slash pdf slash selden dash tuning dot pdf. : http://www.riggingandsails.com/pdf/selden-tuning.pdf    Our rig is discussed on pg 39 and 40.

Sorry, but I have no more suggestions - perhaps others will.

Best regards,

Craig Briggs - SN#68 Sangaris