Mark,
With your 'option 1.', applying heat from water side it will be difficult to heat the tube-ends and tubeplate in the middle of the 'nest' of tubes as they will be sheltered from the direct flame. The advantage of this approach is that you can check penetration of solder easily from the 'firebox side' after soldering and the finished article is perhaps going to look neater. However, who will look inside your firebox??
If you prefer this approach you could get-round the issue of heating the tube-ends etc., if you soldered the tubes in stages/groups, soldering and subsequently inspecting those in the middle first and then adding the next layer outwards, etc. making sure at every stage that the flame could easily reach the unsoldered tube-ends. This would of course involve a number of re-heats with the issues around that. As a precaution it may be worth covering those tubes already soldered with flux.
With your 'option 2.', when applying heat from the firebox side it is far easier to heat the whole end, including the tubes uniformly. There is a risk of 'burning' (overheating) the tube-ends, however, in my opinion this risk is overstated – let's face-it, once you see the solder flow you will remove the heat.
Your option 3., soldering the firebox sides in a separate operation merely involves cleaning and a re-heat of the job after soldering the tubes to the firebox top. The advantage is that it will be a little easier to heat the tube-ends and firebox end and see what you are doing without the side 'in the way', and subsequent inspection and any additional repair of poor penetration at tube-ends will be easier. Heating the sides and firebox-top (with tubes attached) should be relatively easy with good flame access to locations needing soldering from the 'outside' of the job/structure. So, easy flame access throughout the job.
Downside of the separate operation to attach firebox sides is time and gas used in reheat and risk of re-melting some previous soldering of tube-ends to firebox roof. However, a layer of flux around the tube-end soldering should mitigate this eventuality provided that you are sensible.
For any of your options above you would not need different melting-point solders – unless that is an approach that you prefer, nor any heat protection beyond perhaps a few pieces of scrap metal strategically placed to protect tube-ends etc., from direct flame during reheats.
Ya pays yer money and takes yer choice. I would (probably – not having seen the boiler in detail) take option 2 as recommended by Fizzy (he has a lot of experience and knows what he is doing!) = heat tube-ends/firebox top from firebox side with (structural) firebox side in position. One heat for whole job.
Regards & best of luck!
Phil.
Edited By Weary on 22/11/2021 11:54:34
Edited By Weary on 22/11/2021 12:15:56