I'm sorry to give a negative view ... hopefully an expert will pass by and correct my opinions ...

1. Radweld should be avoided at all costs, it can do more harm than good by blocking internal pathways, and it disguises real problems that return to haunt you.
2. By the sounds of it you have only topped up with plain water, and quite a bit over time. This will do nothing but damage to much of the coolant system as the 50/50 antifreeze mixture does not just protect against frost, it is a major corrosion inhibitor as well.
My opinion ?? spend an afternoon with the engine running and some coloured paper towel wiping EVERY part you can reach, where the towel changes colour as it gets wet may well help trace the leak. Once found/fixed flush the system thouroughly and fill with a 50/50 mixture.
3. Oil may well be cam cover gaskets if the breathers have not been cleaned.
Read the guide and clean them .. couple of hours work and if I can do it anyone can. Then do the cam cover gaskets. I did both breathers and gaskets in 2 days
4 Poor fuel consumption, to check for air leaks with spray can of carb cleaner, ( you 'll have some from 3

) spray around the engine whilst it is running, when the note changes you are probably close to where the leak is.
HTH ... good luck