Hi Gary...you are sure making some progress on the Prestina project!
It seems that one of your biggest issues right now is stuck bolts and the breaking of same, since each one of these broken bolts is going to give you a big headache down the line, but you likely already knew that. I think that you need more heat to get these loose, since to remove a bolt with a Detroit wrench one has to get the bolt head more or less cherry red, but since you have broken a few and there are not a lot more left the next issue is getting the bolt studs out! The more patience I apply to a stuck bolt the fewer broken bolts I have. I once worked on a La Duchessa and with every part, irreplaceable part I might add, I sweated that I would break a thread or bolt and called on every bit of patience and presence of mind to just physically ramp the power to this mental point of being below the break it threshold and not go beyond until it was all apart. You know how long it took me to get that broken down!
I would look at that boiler plate and ask myself the question "WHY do I need to remove this part?" and if the answer is "cuz" then this may be enough but take, for example the steam and water valves. These can be rebuilt without removing them from the plate so there is not much of a reason to remove them but for completeness (and possibly another broken part).
As far as tools go, I am pretty tool heavy, full size abrasive blast cabinet, smaller soda blast cabinet, air compressor with 120 gallon tank, welders, oxy/acetylene, air impact tools, powder coat booth, on and on but for me, likely the best tool that I have, and this is in essence a hobbyist tool, is a Proxxon drremel type tool.
Proxxon makes model making and hobby type small format tools and the Proxxon dremel is a DC power tool with a transformer and about any and all bits that you can imagine. I constantly use the wire brushes for both inside and outside of threads and general tight spot cleaning. The tool and attachments are very high quality and the rotary accepts drill bits as well, and the bits are actually like milling cutters for getting in to drill again in tight spots.
I also have a Proxxon drill press, again, small for precise work. The cost of these tools is far less than equivalent quality large scale and I find them to be used much more than the big stationary tools. The Proxxon drill press can be carried in one hand and stored on a shelf. I have actually clamped the Proxxon drill press to a boiler just like you are working on and avoided a lot of hassle drilling out broken bolts and the like.
The big power tools are real macho and make one feel totally empowered but to get a good quality tool they are really expensive and the cheapies are almost more trouble than they are worth.
On the powder coat issue...yes, powder coating is a very good coat but many good rattle can jobs exist on frames...just use good paint.....when you turn any part of your resto over to someone else you get what they do and once that coat goes on it is tough to get it off but if you personally screw up some paint then a bit of sanding and reshoot can put it right...and it is just the frame after all and your maximatic was originally painted, not powder coated. Blasting off the rust on a frame is nice though, but there are other methods which are basically more labor intensive.
One last thing.....you can probably break that group loose with a simple putty knife or just whack it with your trusty mallet.