FWIW, here's my opinion on your problem. First of all,
I have gone around this block twice (with 2 homes) and it
is no fun.
I think the first roofer is right.
On the venting, I would recommend a minimum of adding
"ridge vents" near the peak of your roof and inlet vents. If
it were me, I would go all the way and also add an attic fan
(or fans, depending on the size of your house) and additional
inlet vents. If you have A/C, your attic fan will save more
on your A/C than you spend on running the attic fans themselves.
Your roof will last longer, too. Properly vented (with passive
your attic will hit about 140F on a super hot day. With attic fan,
you can get this down to about 110F.
I agree that you need R30 in the ceiling and that it should be
the first priority. Installing it is literally a pain. Get some
of plywood that you can lay across your rafters. This will ease the
pain. Keep clear of your light cans by 1 foot. Also, make sure
that you don't block any of your eave vents.
If at all possible, do your insulation when the roof is off. That
is a lot more room to move, you can stand up, rather than crawling
through fiberglass, and you will have better ventilation so you won't
be wallowing in fiberglass dust.
If you have to do the job piecemeal, then get a jumpsuit and tape the
wrists and ankles shut. Wear gloves. Wear a respirator. You are
going to be wallowing in fiberglass and swimming in fiberglass dust.
Installing is a 2 person job. Once you have crawled to an area, you
need a helper to "feed" you strips of insulation.