Have I done the right thing. Cadence/crank length.

Hi All;

On the 8th of August last year I was cycling home from work, and at the top of a hill I suffered a complete cardiac collapse, brought on they think by double pneumonia,coupled with undiagnosed narrowed blood vessels. I woke up 6 days later in Coronary Care having been fitted with a set of 5 stents (keep the vessels open for the uninitiated) around my heart. I was very very lucky, and owe a huge debt to quite a few people. Anyway I have undergone rehab, and am back on the bike.
I have had to change my way of cycling. I used to like to feel some resistance even on hills, and used a slowish cadence. I know this is not the modern thinking.
I have to keep my heart rate below 118 BPM, which is nothing really as you will all know. I also have to try not to get too out of breath. I have been using my sons TREK mountain bike, as my own road bike got slightly bent by a van. :O , and have learned to cycle in a new way for me.
I now use a constant cadence of about 65, and keep as close to that as I can, whether, up hill or on the flat. Any faster and I don’t feel it a physical struggle, I just get out of breath. Any slower and I get lactic build up in my legs due I think to not being relaxed, and forcing myself to pedal slowly. Downhill I allow myself a bit faster. I listen to my body and change up or down accordingly. But do not use the granny, as I cannot stand the fast pedalling and getting nowhere, even on hills.
So….. question 1. Could I achieve what I need to do another way, or is it a case of well done, you have found a way to get back into it?
Question 2….I have rescued the bits I could off my old bike, and some friends gave me other bits, and I bought some other bits. The upshot is I now have a new/s/h bike in bits ready to build. Based around a Specialized Ruby Comp frame, given to me by a friends wife, who is now the proud owner of a custom built titanium. I know the Ruby is female specific, but as I am only just over 5 foot, it fits all my bike fit specs, (short legs 26.25 inch inside leg, longish body for my size) and the saddle will still only be a bit higher than the handlebars! My question relates to the crank length. I bought an SLK compact crank set and decided to use 175cm long cranks, in order to compensate for the slower cadence. This flies on the face of current thinking, my bike fit specs, say 160mm would be correct for me. I think that less saddle setback will compensate for my foot being 15mm forward from ideal. The question again, have I made the right decision?
Third question. Should I have gone for a triple?

Sorry about the post length. I am a recreational cyclist/commuter , that cycles to keep fit for skiing.