If you are happy with the Saucony fit, then the Guide sounds like the shoe for you. I picked up a pair for £55 just before Xmas with one of the regular discount codes I get from one of the "RW Shopping partners" listed to the right of this web page (sorry, can't remember which one).
I've been using the Saucony Mirage for the last few years, which is similar to the Kinvara but with a bit of support, and the Guides felt comfortable straight away. I've only managed a couple of runs in them but they fit well and no teething aches/pains etc.
all good advice here, but I just spotted something in the OP about running 2 tempo runs per week.
It may just be the term that you are using, but most plans would be 1x interval and 1x tempo session per week. If you are doing two tempo sessions (i.e. a few miles at 10k to HM pace to raise Lactate Threshold) then maybe consider swapping one for an interval session(e.g. 6x 600m at 5k pace with easier running of 300m between them) to boost your VO2 max. Another vote for pfitzinger & douglas who give a much better explanation of this!
you'll get different advice from different people as we are all ... different.
Listen to the advice, but experiment on your long runs to find what works for you, and most importantly what doesn't upset your stomach. Running a marathon tests your gastric system along with the rest of your body.
Personally, two mistakes I've made that may be relevant to you:
(1) London has water stations every mile, so be disciplined and work out prior to the race how frequently you'll take a drink and resist the temptation to drink too often, as its easy to take on too much water.
(2) On my last marathon, I took my own gels but then took some isotonic drinks from the stations. This was a mistake as the combination was too much for my stomach, causing stomach cramps etc. I'd trained with gels and plain water, and should have stuck to that.
How important to you is getting a good time in Berlin?
The taper is important for enabling your body to fully recover from the training effort but not lose the benefit of the training. The difficulty is (like everything else) that people react differently to it. You may find the traditional three weeks is too long for you, and therefore following a two week taper after the HM race might work (looking to 75% then 50% distance in the two weeks prior to marathon). But this is trial & error and does have a potential impact on your marathon performance.