Motherease nappies can be a bit prone to this sort of thing because, unlike most other nappies, they have a short thick pile. The stay-dry insert is particularly prone to stains IME.
Sounds like you are doing everything right with regards to washing. I only wash every third day (unless the weather is warm -then I wash more often).
I use sandy's for my DS. He is 15wo and BF so we have the same poo issues as you. I use big fleece liners to help keep the stains at bay. You do get some stains through the liner but no way near as much as you would without of course. I also like them as they keep his bottom nice and dry. tbh I wouldn't worry too much about the stains - most are not premanent and even old ones will come out on a bright sunny day. Flibbertyjibbet is right that hanging them outside even on a drizzly day will help (and the rain water will make them nice and soft) so if you can spare a few on the line not getting dry give that a go. We are fortunate enough to have a conservatory where I can dry my nappies (it's a full time laundrette in there really ) and the sun coming through the window helps bleach my nappies (and my conservatory suite covers ) so even drying them by a window will help a bit.
The pong I think is more of a PITA. Chances are they aren't drying quickly enough indoors and that makes things worse. The smell is caused by bacteria still present in the nappies after washing - like flibberty said - if the nappies hang around in a damp environment the bacteria multiply and you get even more build up of the smell-causing monsters in the fibres (the recent warm, damp weather has made this much worse). What you need to do is get rid of the little blighters every now and then and you won't have such a problem. I posted how I do this on another thread so I'll paste it here...
Clean the filters and wipe out the rim and door seal of your washing machine. Do a service wash if you think you need to (empty cycle / hot wash or whatever your manual suggests).
Then...
Buy 500ml WHITE MALT vinegar (it's cheap stuff)
Put CLEAN nappies in nappy bucket
Pour all of vinegar over nappies and top up bucket (almost to top) with warm or cold water
Leave to soak overnight
Plonk nappies in wasing machine next morning and wash at 30 or 40C with a full dose of BIOLOGICAL powder
(I also add some warm water with several - up to 10 - drops of lavender or tea tree to the softener compartment but this is optional)
Once the wash cycle is over then rinse once or twice to get rid of any excess powder.
This has worked every time for me so far - it's not something you should have to do often.
I have to say I do disagree with flibberty on one thing though (really sorry ) I haven't found bamboo or hemp to be as bad for harbouring smells. Microfibre is prone to smells because it has a very high surface area (and thus more surfaces for smell-causing bugs to colonise). Bamboo does have natural antibacterial properties so that helps with the problem but bamboo nappies are often short pile too so harder to keep clean. Older, well used bamboo nappies tend to be as likely to harbour smells as cotton IME. I've used very old, very well used hemp nappies for several years and only had to do the vinegar / biological powder routine once on them.
If you are still awake by the end of this post - well done, I'm impressed!