Varnish suitable for both oil and acrylic

Hi Ron

I am unsure about exactly what you are asking. Do you mean you want exposed canvas on a painting that has both acrylic and oil paint on the surface? If so a clear coat of acrylic dispersion medium on the canvas would be good followed by the acrylic dispersion paint, and then the oil paint. After the whole was allowed to dry for an appropriate period of time the whole could then be varnish if desired, see below. It may be helpful to read this document in our resources section:
https://www.artcons.udel.edu/mitra/Documents/MITRA_Varnishes.pdf
If you are asking how one could varnish a painting with exposed regions of oil paint and areas of exposed acrylic dispersion paint AND protect the acrylic dispersion paint from theoretical damage from cleaning, that is a separate issue.  I agree that a sacrificial coating of clear acrylic dispersion medium over an acrylic dispersion painting is a good idea when it is to be covered with a MSA, Regalrez, or similar aliphatic/aromatic soluble varnish. This does not work very well in a work where portions of the exposed painting are acrylic dispersion and others are oil paint. The acrylic dispersion medium would have poor adhesion to the areas of exposed oil paint.
I would recommend that if you want to varnish such a work that you either simply coat the whole with the MSA or Regalrez, etc. varnish.
Alternatively you could give the whole a thin application of an acrylic solution varnish (resin dissolved in solvent) before applying one of the above. For instance, B-72, while not a “pretty” varnish, could withstand the solvents used to remove an MSA or Regalrez varnish if the painting needs to be cleaned in the future. B-72 is one of the most stable materials available. Its use would require that the work be allowed to dry for a good period of time as it is not soluble in solvents less aromatic than pure Xylene (100%).
The final varnish actually provides most of the aesthetic, saturating effects and a surface coating of MSA or Regalrez over B-72 would create a very saturating effect if desired.
Recording your use of multiple varnishes somewhere on the painting (again see our resources section) would allow future conservators to choose a cleaning system that only removes the surface coating and does not completely remove the B-72.
Having written the above, I would probably just go with the first option.
Of course, others may have other, better answers to your question.

What you propose is fine. There would be no reason to remove the GAC at a later date. It is not really known to yellow appreciably at all.
The idea of applying a sacrificial, not necessarily removable, layer od acrylic dispersion medium over acrylic dispersion pigmented paints it to allow for potential (and only theoretically potential) biting into a pigment coat of acrylic dispersion paint. This should not be an issue at all in the scenario you describe. Any minute abrasion (etc.) of the clear acrylic dispersion interlayer caused by cleaning would be filled in by layer varnishes.