Reversing from Platform 1 at Harrow & Wealdstone is performed at least once a day. The problem with the move is the driver needs time to shut down one end of the train, walk to the opposite end of the train, and set up that end, before departing. That's a minimum 5 minutes, then there's the time to reach the crossover to the southbound line, then re-set the points to let the next northbound into the platform. Therefore you need at least 8 minutes between trains!
LU trains running to Watford Jcn would need more rolling stock on the Bakerloo, there aren't any more '72 stocks.... and so nothing can or will happen on that front until New Tube for London stock arrives in the mid 2020's or later.
The other problem with running LU mixed with LO is that LO trains are faster than LU trains, so a mixed service all the way to Watford may not be practicable, so it would have to be all LO or all LU. The other restriction is that north of H&W there is a 4 minute headway, which is fine until you remember the Met extension!!
No extra platforms needed at Watford Jcn. - there's 4. 2 platforms is plenty to turn a 5 minute service. Proof: Elephant & Castle turns a 3 minute service with 2 platforms.
What would work far better is to properly inter-schedule the London Midland services with the existing LO services, so that overcrowding on the LO is reduced, though this may then cause overcrowding on London Midland. The Met extension may give some relief.
I have to say travelling myself daily from Carpenders Park as I do, I use LO south to QPK and switch to Bakerloo to Embankment for my final destination of St James's Park. But travelling home I route via Embnakment to Euston, hopefully for a London Midland express to either H&W or Bushey and then LO. Why do I do this? I like to have a seat!! Not necessarily the fastest route in either case, but much higher chances of a seat in peak periods.
The big problems with overcrowding are between Willesden Jcn and as far as Wembley Central, maybe more London Midland trains need to stop there. I've been on peak LO northbound services which were standing room only as far as my own station, that's far too far. The LO service really needs to ramp up in the peaks and it doesn't. Fixing that and re-timing LO and London Midland to give better interchange times at H&W and Bushey would probably even out the overcrowding to 1 or 2 stops.
As correctly pointed out, anything that increases the number of trains required on the Bakerloo line is going to cause a stabling problem. South extension, north extension or increase in frequency in the peak, all need more trains, and there's nowhere to put them!!
Elephant's sidings are not usable due to Health & Safety restrictions, that is the current has to be turned off before the driver can exit the cab, precluding their use. One train is already stabled in Elephant's platforms overnight, ditto for Queen's Park. That's not for convenience, there really is nowhere else to put them!! So whatever extension is planned / goes ahead, needs to include more stabling space, as obviously extensions need more trains.