I wish I had 10p for every handful of assorted debris I have removed from under various floors when boards come up. As you say, Freddy, it is all too easy for previous 'tradesmen' to leave everything there (sometimes including tools, pipe fittings and other useful items!).
As to replacing your damaged floorboards, much will depend on whether they are to be covered afterwards (fitted carpet; vinyl; laminate ...). If so, why worry about the different width - if the thickness is the same, surely you can mix-and-match with reusable removed boards and supplement in a suitable place with new ones of whatever width is available (cutting a narrow one if necessary for the fill-in)? You may need to remove a few more boards to achieve the optimum repairable area. And I question the real need for t&g, unless draughts or dust are a particular problem. OK, you may end up with a bit of a patchwork, but if it's solid and covered, so what? You are lucky in that you live (I assume) in a relatively modern house with boards that can be matched from stock for thickness - my Victorian house has boards that are about 3mm thicker than anything commonly used today; I always keep (or scrounge from elsewhere) any decent boards that match for thickness. Another suggestion is to use chipboard (preferably P5 t&g) in 18mm or 22mm if this matches for thickness - a single panel might get you out of trouble in an area that is unlikely to need lifting again. I nearly always screw down floorboards; 1.5" or 1.75" No. 8 csk screws are ideal and allow for subsequent removal with minimal damage to boards or anything under them.
A router is an extremely useful tool; I would not be without mine and am certain you will love it if you get one and realise its potential; very little practise is needed. It does not have to be expensive either; 28295 from Toolstation (£49.98), SF 49312 (£49.99) SF 31457 (£35.99), or possibly Argos 711/3914 (£39.99) should all be fine for anything other than heavy repetitive work. An extra set of 0.25" router bits is handy; again not expensive (and much cheaper than buying individually). Mrs Freddy should be instructed to put these items on your Xmas list - NOW! With the supplied fence and a Workmate, you can rout most things - I do have a small table but rarely use it - too fiddly and not always easy to use in its fixed workshop position. You should be able to rout board edges (if you have to) using just the fence from one side, and possibly without needing to plunge cut - I rarely 'plunge' as I find it more difficult to control than taking (if necessary) several cuts at different fixed depths. Just remember to allow a little extra board at each end so that the router/fence still has something to bear against at each extremity of cut.