Most appropriate synthetic for halyard / staysail luff

[svaletheia]

Hello all again. Your local noob is hard at it here with some more questions for you experts.

Thanks to this forum's expertise and some help over at Colligo, I think I am going 100% to Dynex Dux for my rigging instead of stainless wire. The timing all depends on money, really - I'm hoping to do this in December. I'm pretty psyched about it. Also, watching the splicing videos for Dux, it looks pretty darned straightforward compared to splicing braid or double braid.

That said, now I'm considering using a synthetic luff for my staysails instead of the heavy and nasty steel cable in there now, and would like an equally non-stretch halyard material. My research hasn't turned up a lot on using synthetics as halyards, and my reading up on kevlar seems to indicate that brittle fibers don't like being sheaved frequently. Is there a significant difference between how various synthetic types (Amsteel, Dyneema, Spectra, Dux) handle being a halyard and anything I should know before deciding?

Second question: how do you size a halyard for a given load? I understand the pre-tension being easily calculated, but how about the dynamic load as the sail is loaded by the wind? For a jib, I can see some part of that being borne by the stay and some part by the halyard providing increased tension, but for a staysail, it seems the halyard gets the entire brunt of it. Some help in sizing or calculations would be useful.

Third, I see a lot of discussion about how slippery these lines are and how small you can get them for halyard duty, and I'm reading the thread about splicing a cover over the lower portion with great interest. What is the "approved", or "standard, proper" way to get a thicker, higher friction lower portion of the halyard that retains 100% of the material strength from the cleat to the sail head?

Lastly, I'm definitely not working on an unlimited budget here but the synthetics seem to be relatively price competitive, load/stretch for load/stretch with any other rope type out there. What other applications are they particularly well suited for? (e.g. given how slippery they are I can't see them easily used as a vang with a cam cleat, but I could definitely see a topping lift or lazyjack application - though they might be damned pricey for lazyjacks and I might be better off with poly rope). Some expertise here would be very helpful as I plan the budget for the re-rigging. How do they handle being used in a traditional spinlock-style rope clutch? I'm aware of their usefulness as shackles and hanks, but any comments on their suitability, usefulness, and application beyond that would be useful.

Thanks so much for helping me sort out the facts from the myths that are so prevalent out there about this stuff, and for your patience as I learn.