In practice, miners & mining pools aren't actually building blocks & they never touch the mempool nor build/verify any transaction besides the coinbase - bitcoind handles all that. Handling Fees, signatures, etc is the job of bitcoind. By the time the pool process received the getblocktemplate() response - the block is already built & verified - besides the coinbase.

What pools do is keep track of miner stats, handle their networking & if a share reaches the target - tell bitcoind about it, which is done via the stratum protocol, which bitcoind has 0 idea about & is much more resource intensive than block verification/construction.

Everything you need is in bitcoincore source code & it's the easiest to work with sources you'll find on this topic. It depends on a few things though.

OpenSSL is a great option - the problem is OpenSSL is very difficult & unforgiving to work with. What "low level" functionality isn't in openssl? For all the eliptic curve maths, openssl big number library is going to be faster for high performance than bitcoincore's arith_uint256 classes for example, the same can be said about hashing (hmac, sha256, etc) - but at the cost of being significantly more difficult/cumbersome to work with, accompanied by less documentation. Bitcoincore big number addition for example is 2 or 3 lines of c++, openssl would be closer to 10 with the BN_/BIGNUM* interface.

Bitcoincore isn't "c/c++" though, at least the parts you'd need, it's 100% advanced c++ with many templates/lambdas/multiple inheritance/etc. If your c++ is rock solid - you'll find bitcoincore very pleasant, as a breath of fresh air simplifying the advanced, if your c++ isn't that good - bitcoincore will be a nightmare. OpenSSL is 100% old school C89, you don't need to know what a template or class is to work with it, but openssl won't hold your hand in the way bitcoincore will in terms of memory management & knowledge of advanced maths required (Especially for elliptic curve stuff - signing a TX in openssl is completely diff, IMO more complicated, than using libsecp256k1 in btcore).

I'd say the determining factor is you, and whether you do better in C or C++, but in general the proof of work crypto world is advanced c++, all the daemons, miners, pools, proxies, etc are using advanced c++ so I'd recommend sticking with that. 

Bitmain definitely knows what they're doing & if someone can optimize anything on that board - it's them. They're not obliged to tell you anything & can use whatever software/hardware they want, & then you can do what you want with it.

That aside, there's a lot of info/documentation on their asic chips lately. Tons of folks are soldering them to their own PCB's. We're already seeing many projects with one or two asics on small pcb's controlled by some small mcu or via serial by a pc, next they're going to put 5 on a board, next 10, then you're going to have full fledged open source s19 type machines. It won't change anything though - The vast majority of the cost of a sha256d miner is in the asic development/wafer printing - even if you hand crafted a shad256d miner asic by asic, resistor by resistor, etc, custom controller board, etc with your own soldering iron - the cost would be similar to MSRP for a s19, for example. Bitmain is selling those machines for very, very close to their production cost. Just in case you're thinking it's cheaper to build your own miners.

To answer your question though, how much to hack a fan control? That's more of a reverse engineering job than development, reversing costs are generally much cheaper than programming. I'd say not as much as you'd think, plenty of forums with lots of talented reversers who'd probably work with you for a few thousand US. If you want a custom pcb controller board with tested, quality software, you'd need to start adding 0's to the price.

Nicehash is going to be shutting down soon, we had a great run but the tldr is, creating a spot market for PPS is technically incredibly difficult - no one else besides them can do it. Their rules were/are constantly changing & they're so rich now I think the 1 or 2 rockstar devs they have are just tired of dealing with all that. We (buyers) generally will pay more than their new lottery mining business (which is where a significant portion, sometimes all, of their hashrate & business efforts go). It happens almost daily now where there are stacked standard orders sitting empty, unfilled, offering to pay 50-100% more than their fixed lottery orders. Meanwhile they pay bloggers to write about how nicehash pays the most & put restrictions on what orders their stratum servers fill & do everything to drive hash price down. Then when you add on the regulation aka harassment they're suffering (because they operate in one of the most restrictive jurisdictions on earth) they're loosing incentive to even exist. They want an easy, cushy job mining a handful of the top 50-100 coins on a low risk, lottery basis instead of the technically complex, nearly impossible to get right, high risk, high reward PPS spot market they're running.

Does no one here run GPU farms anymore??? Guys please contact me if you have GPU farms, I don't have a pretty front end because running nh bots you don't need them - but I'm polite & easy to deal with & will pay up front (if you've got a rep here - I'll pay you directly up front, otherwise escrow until we build trust) & make you an offer you cannot refuse on GPUs - you will probably end up with free electricity - contact me! PPS payout in BTC (or whatever you want!), legit offer, looking for long time partners!

Signed - soon to be former longtime nicehash client

Hey guys, I run a small product/pool & need kawpow miners ASAP. I can pay out in BTC (or any major coin) @ nicehash fixed spot price or whatever your favorite calc price + 2% on a PPS basis (I assume most miners are looking for stable payouts but I can offer higher, but more volatile payouts). I'll pay highest for up to 50 GH - show me your paystub from the highest paying pool - I will beat it by 2%.

I have software to monitor hashrate, I have no problem using a trusted escrow service & paying up front on an hourly/daily/etc basis -- please contact me if you run a GPU farm - - thanks!