Is Mongoreplay Included in MongoDB 4.0 Community Version for Windows?

As an answer to your first question, both tools (by default) will just walk the _id index to fetch the data and then write it out to disk. So, yes, both tools will similarly impact your working set which is why I would generally recommend running them against a secondary (preferably a hidden secondary if possible). I'll echo Stennie in the comments here and recommend other backup methods if you are dealing with large amounts of data.

For the second question, I assume you are looking for a mongodump equivalent of the --fields option from mongoexport to only dump out specific fields. The query option can be used to filter results, but it cannot be used with a projection (to select the fields returned) - this is a feature request that is being tracked in TOOLS-28 but is not yet scheduled. As Stennie also mentioned, the other option here is to write a custom exporter that fits your needs (and again, I would still recommend running it against a secondary to protect your working set).

As at MongoDB 3.4, no plans have been announced to make the In Memory Storage Engine available in the MongoDB Community Edition. I suspect this is likely to remain as an Enterprise-only feature for the foreseeable future given the more niche use cases for in-memory storage, however the definitive source will be the mongodb.com website and any future major release announcements.

FYI, you can subscribe to driver/server release announcements via the mongodb-announce group.