Our recommendation would be to do the massonry work first and then roofing for a few different reasons.

1.This gives the masons freedom to step and walk on the roof without causing damage to the new shingles. 

2. You will want to make sure the new flashing that is intstalled around the chimney is installed correctly, accounting for any masonry changes and is not damaged due to installation.

3. Ultimately, that the new roof is not damaged and that the warranty does not get voided. 

Brick mason

You can proform both jobs at the same time if you are good at what you do. I have in the past grinded a Grove in the brick chimney all four sides then used a brake to bend a little half inch lip on the top of my flashing metal flashing into massonary Grove then caulked you will never ever have to worry about again in your lifetime Only reason I did massonary work first is because I didn't want to get my shingles all dusted.

For a great number of years, I was a mason and chimney sweep before rolling out to full bathroom and ktichen remodeling.  It still is my first love.  Please have a certified, licenced chimney sweep out to look at the chimney.  /http://www.ncsg.org/ can put you in contact with a certified chimney professional in your area. Please do not use it until it is checked. 

Philip Anderson

Berkeley Ca

Masonry work first. Best, Ken

There are a very wide veriety of gas fireplace inserts.  

1. Check with the city to see what you can install.  Some cities are now requiring sealed units

2. Do not try runing a gas line by yourself.  Hire a licensed plumber to do it.

3. http://www.csia.org/?gclid=CL_Qk-SZ0dACFZBefgodOrUNRA Contact a CSIA certified chimney sweep to help you install. 

Thanks

Philip Anderson

HDR Remodeling Berkeley Ca

No, but have you thought about contacting a manufacturer and asking for a local referral. Sometimes it is an unknown fact but home remodeling product manufacturers have authorized dealers and installation crews listed on their site! This guarantees the best match. We always explain to homeowners having a certified installer is more important than price! Residential Renovations works very hard to make sure all installers from roofing, siding and windows to decking, sunrooms and beyond is installed by professional crew members who are trained as the manufacturer warranties. Good luck with your project! You can visit our website at www.ResidentialRenovations.com to learn more about Residential Renovations - serving Ohio and Michigan homeowners!

http://www.ncsg.org/  The National Chimney Sweep Quild is a great source for licenced and certified professionals in your area. If they don't do the work, they will know someone in your area who does.

Philip Anderson

HDR Remodeling

Berkeley Ca

Each case is unique but in general, the chimney should be done first. Special attention should be paid to the roof flashings around the chimney.  If the flashings are behind the stone then make sure they extend far enough into the shingles so the new roof can be flashed properly. If the flashings are surface mounted and there is a counter flashing then the roofer should have no problem making the chimney base watertight   

Hard to say based on the limited description available. Is the fireplace constructed completely of masonry? or is there an insert in the firebox? Is it framed walls with stone/brick veneer? Is the chimney a metal pipe or clay flu?

If you haven't used it and it is cracking, I would suggest that the foundation was possibly inadequate and it is settling.

Or the materials used were inferior materials, or the mortar wasn't a quality mix.

There are many factors that could be the cause. Hard to say based on the limited information.

I will say that converting from a wood fireplace to a gas fireplace shouldn't be much of an issue. Converting from gas to wood on the other hand is virtually impossible without major remodeling and expense.