My events are very similar. Although I have a 40km in May and 80km in September.
Here’s my thoughts.
Q1: it depends on how AnT inclined you are or where you feel you can improve more. For instance, if you feel you need to build more AeT then perhaps extend the build phase. But also listen to your body after the 50km – it might give you a good chance to take a week or two of light/unstructured training. I would suggest taking some time after the event to allow your legs to recover and your mind to decompress from months of training.
Q2: don’t get too hung up on low volume weeks, a week every now and then won’t make or break your training. Just try and not over do it before or after these weeks. Months and years of consistency is what’s important. The biking will still give you some stimulus but don’t forget that amount of biking might need some recovery afterwards (does sound fun though). don’t worry about if there are a few more rest/low volume week in a block – better that than more build weeks.
The build weeks after will depend on your history. Personally, I would keep everything the same, depending on how the week goes I might perhaps shorten the long run. Just play it by feel. it’s better not to over push which could have a negative effect on future training.
I assume these zero running weeks for family holidays etc. these too will fatigue your body, sadly the body doesn’t differentiate, To it stress is stress, again allow your body to recover.
It’s easier said than done but I hope that helps.
Good luck for your training and races!