I have a early 1900's ranch home that had an addition with a reverse gable roof added on to the back of the home shortly after the original structure was built. A hole was cut into the back of the original roof to allow access between the old attic and new. The problem is attic ventilation in the addition. The addition has no soffits and therefore no intake. A new roof along with ice n water shield and ridge vents were installed on the original structure and the addition 10 years ago. The unfinished attic has about 4" of cellulose and I want to add more before the snow flies this year. Due to the lack of insulation and no intake venting in the addition, ice damning has been a problem over the years along with signs of old mold or moisture problems(gray dusty spots) along the roof decking.
My local insulation contractor suggests that due to the low pitch (4/12) of the roof, adding more insulation alone will not give me enough r value at the eaves to prevent my ice damning concerns. His suggestion is to apply 2lb spray foam along the bottom of the roof decking about a foot from the outside edge of the eaves the entire length.

To address the intake concerns, he suggested adding gable vents (3) and make the cut out between the two areas larger to help increase air flow in the attic. . The original or front of the home has good soffit intake and exhaust and there are no signs of mold or moisture problems.
So these are my concerns.

The ventilation in the original structure appears to work just fine. Why add gable vents to this part of the home?

In the addition, with one new gable vent, should I block off the first few feet of the ridge vent at the gable end to try and get more air to pull thru the addition?

Should I block off the space between the old and new areas so as the two areas work independent of each other?

I know that soffit vents and ridge vents combined are the best, but I am not comfortable installing products such as "sure vent" to create an intake due to the ice damning problems of the past and the fact I am adding the spray foam at the eaves.