1 - Yes, it can be retrofitted depending on the style you choose & if that style fit your soffits. For example, an Aluminum Fin type would need a 6" soffit or eave & would require a 2" slot to be cut-out, because the fin section is recessed & must sit into the slot. Vinyl Holes type would need 11" to 12" to look professional but you can cut out or drill whatever openings you want as it just sits flat.
2 - Gable Vents work regardless of anything else or nothing else, but I would ditch any older in-the-field roof work vents as the roofing my not be pliable & break off in pieces. Gable vents are regularly enlarged & re-shaped to meet the desired ventilation. I've seen the entire gable turned into a vent. I think enlargement is your 2nd best now answer.
3 - I'm sorry I don't know the cost difference, your local roofers will vary & there's more labor in Boxes so they may be equal out if Ridge Materials are more expensive. But, Ridge Vents would be best for an old roof due to brittleness of the field shingles. Plus, Box Vents are quite small & due to their very small amount of penetrations they don't work well to start with.
4 - Ridge Vent re-use is a little tricky. The issues are nail removal & then re-nailing. You don't want to pay your roofer to waste time ripping out nails at 4x's the price of new ridge vent (dramatization) nor do you want him nailing in old holes where a tight attachment may not be achieved.
5 - Keep the existing Gable & Soffit vents as they are & install a Thermostatic or Switched or Timed fan in one of the Gable Vents to beat all of the above. This way you can cool when all of the above may not work to your liking without the direct solar gain (rainy, overcast, nighttime, snow covered). I think this is your overall best option with #2 being the closest behind.