Merit of standing Alu clad timber on full Alu cills - arbo

[gcm]

One of my potential installers has suggested the following for alu clad timber to address a site difficulty with suspected insufficiently sloping granite cills.  Is this a sensible design tweak or a case of everything looking like a nail if you are holding a hammer.

Situation:

Deep brick apertures, wall batt and block construction behind -  rough granite exterior cils protruding (they are not flat yet could likely do with being steeper).  See photo.

The as is existing timber deflectors in front and the bottom of existing softwood frames have shown the worst paint loss and rot.  Some water may well be being retained on the granite.  It's rough so it runs off when there is a lot (from a jug) but surface tension holds a certain amount

I have been uncertain about the merit of standing the new windows on the existing granite and relying upon the compriband at front timber line given the exposed edge along the line of the compriband seal.  We have also looked at fitting some kind of "stub" cill water deflector (clipped or screwed to the face according to system) which will help run off, reduce direct volume, but not eliminate water retention on cill. Discussion with this approach has been on whether to silicone behind face mounted Alu stubs or not (i.e. making a bucket for water to sit in vs keeping it out).

Have rejected full clip on Alu cills out across and down over the rough shaped granite edge as not a practical choice - too ugly although it would work.
Grinding or even replacing the granite during the swap out was considered but undesirable on cost, logistics, difficulty with existing edges/finish etc.

The fitter's suggestion is to use deep Alu cills for the windows to rest upon installed well back on the granite into the aperture leaving a lot of granite cill in front (appearance OK).  Approx. 25mm of sloped down Alu in window finish protruding out from underneath the elevated window bottom sitting above the granite.  The window being bedded onto the Alu cill behind and forward to the normal compriband weather seal line with some form of wet seal he used the phrase "arbo" and argued a special kind should be used for longer life.  Then normal Compriband around sides/top etc.

Thoughts on these two ideas, materials choice or alternatives to address the underlying constraint with potential water retention.